Liquid crystal display device and method for manufacturing the same

ABSTRACT

Provided is a method to manufacture a liquid crystal display device in which a contact hole for the electrical connection of the pixel electrode and one of the source and drain electrode of a transistor and a contact hole for the processing of a semiconductor layer are formed simultaneously. The method contributes to the reduction of a photography step. The transistor includes an oxide semiconductor layer where a channel formation region is formed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/724,666, filed Dec. 23, 2019, now allowed, which is a continuation ofU.S. application Ser. No. 15/916,362, filed Mar. 9, 2018, now U.S. Pat.No. 10,522,572, which is a continuation of U.S. application Ser. No.15/088,200, filed Apr. 1, 2016, now U.S. Pat. No. 9,917,112, which is acontinuation of U.S. application Ser. No. 14/023,515, filed Sep. 11,2013, now U.S. Pat. No. 9,305,944, which is a continuation of U.S.application Ser. No. 13/227,092, filed Sep. 7, 2011, now U.S. Pat. No.8,558,960, which claims the benefit of a foreign priority applicationfiled in Japan as Serial No. 2010-204930 on Sep. 13, 2010, all of whichare incorporated by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a method for manufacturing a thin filmtransistor and a liquid crystal display device.

2. Description of the Related Art

In recent years, reduction in cost, thickness, and weight of liquidcrystal display devices has been greatly needed.

As one of methods for achieving cost reduction of liquid crystal displaydevices, simplification of a manufacturing process of liquid crystaldisplay devices can be given.

Driving methods of liquid crystal display devices are broadly classifiedinto a passive matrix method and an active matrix method. In recentyears, active matrix liquid crystal display devices which are excellentin image quality and high-speed response have been in the mainstream.

In an active matrix liquid crystal display device, each pixel isprovided with a switching element. As the switching element, a thin filmtransistor is mainly used. As such a thin film transistor, a top-gatetransistor whose channel formation region is provided below a gateelectrode and a bottom-gate transistor whose channel formation region isprovided over a gate electrode are given. These thin film transistorsare generally manufactured using at least five photomasks.

Reducing the number of photomasks as much as possible is one ofimportant factors to manufacture liquid crystal display devices at lowercost. In order to reduce the number of photomasks, a complicatedtechnique such as backside light exposure (for example, see PatentDocument 1), resist reflow, or a lift-off method, which requires aspecial apparatus, is used in many cases. The use of such a complicatedtechnique might lead to various problems such as reduction in yield ofliquid crystal display devices and degradation of electriccharacteristics of thin film transistors.

Further, as one of methods for achieving reduction in thickness andweight of liquid crystal display devices, reducing the thicknesses ofsubstrates between which a liquid crystal material is sandwiched, bymechanical polishing, chemical polishing, or the like, are given.

Mainly, glass substrates are used as substrates between which a liquidcrystal material is sandwiched and thus, there is a limit on reductionin thicknesses of such substrates by mechanical polishing, chemicalpolishing, or the like. Moreover, there is a problem that as thethicknesses of such substrates are reduced, the strength of thesubstrates is lowered and a liquid crystal display device is more likelyto be damaged by external impact. Therefore, it is ideal to manufacturea liquid crystal display device with the use of significantly toughsupports (such as a resin film and a metal film) as substrates betweenwhich a liquid crystal material is sandwiched.

REFERENCE

[Patent Document 1] Japanese Published Patent Application No. H05-203987

SUMMARY OF THE INVENTION

The present invention is made in view of the foregoing technicalbackground. Thus, an object of one embodiment of the present inventionis to reduce the number of photomasks as compared to that in theconventional case, without a complicated technique or a specialapparatus. Another object of one embodiment of the present invention isto provide a method for manufacturing a liquid crystal display devicewhich is thin, lightweight, and less prone to fracture.

That is to say, one embodiment of the present invention is a method formanufacturing a liquid crystal display device, which includes the stepsof: forming a separation layer over a substrate; forming a firstconductive layer over the separation layer; forming a first resist maskover the first conductive layer; partly removing the first conductivelayer with the use of the first resist mask to form a gate electrode;forming a first insulating layer to be a gate insulating layer over thegate electrode; forming a semiconductor layer over the first insulatinglayer; forming a second conductive layer over the semiconductor layer;forming a second resist mask over the second conductive layer; partlyremoving the second conductive layer with the use of the second resistmask to form a source electrode and a drain electrode, so that atransistor including the gate electrode, the source electrode, and thedrain electrode is manufactured; forming a second insulating layerserving as a protective insulating layer so that the second insulatinglayer covers the source electrode, the drain electrode, and thesemiconductor layer; forming a third resist mask over the secondinsulating layer; selectively removing part of the second insulatinglayer overlapping with the drain electrode, with the use of the thirdresist mask, to form a first opening and at the same time, removing partof the second insulating layer, part of the semiconductor layer, andpart of the first insulating layer, which do not overlap with the sourceelectrode or the drain electrode, with the use of the third resist mask,to form a second opening; forming a third conductive layer so that thethird conductive layer covers the first opening, the second opening, andthe second insulating layer; forming a fourth resist mask over the thirdconductive layer; and partly removing the third conductive layer withthe use of the fourth resist mask to form a pixel electrode.

According to the embodiment of the present invention, the step offorming the first opening serving as a contact hole and the step ofetching the semiconductor layer are performed at the same time, wherebythe thin film transistor can be manufactured using a smaller number ofphotomasks than those used in the conventional case and further, thelayers including the thin film transistor can be separated from thesubstrate.

In a method for manufacturing a liquid crystal display device, accordingto one embodiment of the present invention, a base layer is formed overa substrate and in contact with a separation layer.

According to the embodiment of the present invention, diffusion of animpurity element from the substrate can be suppressed. Accordingly, achange in characteristics of a thin film transistor due to diffusion ofan impurity element to a semiconductor layer can be suppressed.

In a method for manufacturing a liquid crystal display device, accordingto one embodiment of the present invention, a side surface of asemiconductor layer (a side surface of an end portion of a semiconductorlayer) is covered with a pixel electrode.

According to the embodiment of the present invention, entry ofimpurities from the outside can be suppressed. Thus, a change incharacteristics of a thin film transistor due to entry of impuritiesfrom the outside can be suppressed.

In a method for manufacturing a liquid crystal display device, accordingto one embodiment of the present invention, a semiconductor layerincludes an oxide semiconductor.

According to the embodiment of the present invention, a highly reliableliquid crystal display device with low power consumption can be realizedby using an oxide semiconductor as a semiconductor layer.

In a method for manufacturing a liquid crystal display device, accordingto one embodiment of the present invention, heat treatment is performedon an oxide semiconductor.

According to the embodiment of the present invention, the concentrationof impurities such as moisture and hydrogen serving as electron donors(donors) of a semiconductor layer can be sufficiently reduced.Accordingly, the off-state current of a transistor can be reduced.

As to an oxide semiconductor in which impurities such as moisture andhydrogen serving as electron donors (donors) are reduced (purified OS),the hydrogen concentration measured by secondary ion mass spectrometry(SIMS) is 5×10¹⁹/cm³ or less, preferably 5×10¹⁸/cm³ or less, morepreferably 5×10¹⁷/cm³ or less, and still more preferably 1×10¹⁶/cm³ orless. Furthermore, the band gap of the oxide semiconductor is 2 eV ormore, preferably 2.5 eV or more, more preferably 3 eV or more.

The SIMS analysis of the hydrogen concentration in the oxidesemiconductor is described here. Because of the principle of the SIMSanalysis, it is known to be difficult to obtain accurate data in theproximity of a surface of a sample or in the proximity of an interfacebetween stacked films formed of different materials. Therefore, when ahydrogen concentration of the film is analyzed by SIMS, the averagevalue in a region in which the values do not extremely vary and aresubstantially constant is employed as the hydrogen concentration.Further, in the case where the thickness of the film is small, a regionwhere a constant value can be obtained cannot be found in some cases dueto the influence of the hydrogen in the films adjacent to each other. Inthis case, the maximum value or the minimum value of the hydrogenconcentration is employed as the hydrogen concentration in the film.Furthermore, in the case where a maximum peak and a minimum valley donot exist, the value of the inflection point is employed as the hydrogenconcentration.

In a method for manufacturing a liquid crystal display device, accordingto one embodiment of the present invention, a first conductive layer anda second conductive layer are each formed using a material containingcopper.

According to the embodiment of the present invention, the use of amaterial containing copper for formation of a gate electrode, a sourceelectrode, a drain electrode, or a wiring connected to these electrodesallows reduction in wiring resistance, so that signal delay can beprevented.

In a method for manufacturing a liquid crystal display device, accordingto one embodiment of the present invention, the upper limit of theprocess temperature at the time after formation of a first conductivelayer and a second conductive layer is lower than or equal to 450° C.

According to the embodiment of the present invention, when a materialcontaining copper is used for formation of a gate electrode, a sourceelectrode, a drain electrode, or a wiring connected to these electrodes,the electrodes and the wiring are not deformed, and elution of acomponent in the electrodes and the wiring due to a thermal factor doesnot occur. Therefore, a highly reliable liquid crystal display devicecan be manufactured.

In a method for manufacturing a liquid crystal display device, accordingto one embodiment of the present invention, a first conductive layer anda second conductive layer are each formed using a material containingaluminum.

According to the embodiment of the present invention, the use of amaterial containing aluminum for formation of a gate electrode, a sourceelectrode, a drain electrode, or a wiring connected to these electrodesallows reduction in wiring resistance, so that signal delay can beprevented.

In a method for manufacturing a liquid crystal display device, accordingto one embodiment of the present invention, the upper limit of theprocess temperature at the time after formation of a first conductivelayer and a second conductive layer is lower than or equal to 380° C.

According to the embodiment of the present invention, when a materialcontaining aluminum is used for formation of a gate electrode, a sourceelectrode, a drain electrode, or a wiring connected to these electrodes,the electrode and the wiring are not deformed, and elution of acomponent in the electrodes and the wiring due to a thermal factor doesnot occur. Therefore, a highly reliable liquid crystal display devicecan be manufactured.

A method for manufacturing a liquid crystal display device, according toone embodiment of the present invention, includes: a step of forming anelement region including at least a gate electrode, a first insulatinglayer, a semiconductor layer, a source electrode, a drain electrode, asecond insulating layer, and a pixel electrode, over a substrate with aseparation layer interposed therebetween, which is followed by a step offorming a protective layer so that the protective layer covers a surfaceof the element region; a step of separating the protective layer and theelement region from the substrate; a step of bonding a first supportwhose fracture toughness is greater than or equal to 1.5 [MPa·m^(1/2)]to the other surface of the element region; a step of removing theprotective layer from the first support; forming a fourth conductivelayer over a surface of a second support whose fracture toughness isgreater than or equal to 1.5 [MPa·m^(1/2)]; and providing a liquidcrystal material between the surface of the first support, over whichthe element region is provided, and the surface of the second support,over which the fourth conductive layer is provided.

According to the embodiment of the present invention, the element regionprovided over the substrate can be transferred to the first supportwhose fracture toughness is greater than or equal to 1.5 [MPa·m^(1/2)].Further, the second support with which a liquid crystal material issandwiched also has a fracture toughness of greater than or equal to 1.5[MPa·m^(1/2)]. Thus, a liquid crystal display device which is thin,lightweight, and less prone to fracture can be manufactured.

A semiconductor device in this specification refers to any device thatcan function by utilizing semiconductor characteristics. A semiconductorcircuit, a storage device, an imaging device, a display device, anelectro-optical device, an electronic device, and the like are allsemiconductor devices.

When “B is formed on A” or “B is formed over A” is explicitly describedin this specification, it does not necessarily mean that B is formed indirect contact with A. The expression includes the case where A and Bare not in direct contact with each other, i.e., the case where anotherobject is interposed between A and B. Here, each of A and B correspondsto an object (e.g., a device, an element, a circuit, a wiring, anelectrode, a terminal, a film, a layer, or a substrate).

Therefore, for example, when it is explicitly described that a layer Bis formed on or over a layer A, it includes both the case where thelayer B is formed in direct contact with the layer A, and the case whereanother layer (e.g., a layer C or a layer D) is formed in direct contactwith the layer A and the layer B is formed in direct contact with thelayer C or the layer D. Note that another layer (e.g., the layer C orthe layer D) may be a single layer or a plurality of layers.

In this specification, ordinal numbers such as “first”, “second”, and“third” are used in order to avoid confusion among components, and theterms do not mean the number of components.

A “transistor” in this specification is a kind of semiconductor elementsand can perform amplification of current or voltage, switching operationfor controlling conduction or non-conduction, or the like. A“transistor” in this specification includes an insulated-gate fieldeffect transistor (IGFET) and a thin film transistor (TFT).

Functions of a “source” and a “drain” of a transistor in thisspecification are sometimes replaced with each other when a transistorof opposite polarity is used or when the direction of current flowing ischanged in circuit operation, for example. Therefore, the terms “source”and “drain” can be used to denote the drain and the source,respectively, in this specification.

In this specification, the term of “electrode” or “wiring” does notlimit the function of component. For example, an “electrode” issometimes used as part of a “wiring”, and vice versa. Furthermore, theterm “electrode” or “wiring” can include the case where a plurality of“electrodes” or “wirings” is formed in an integrated manner.

The term “toughness” in this specification represents the resistance ofa material to fracture. The fracture is less prone to progress even inthe case where a heavy load is applied or strong impact is made as amaterial has higher toughness, and the fracture is less prone toprogress when a flaw produced in part of a substrate acts as a startingpoint, for example. The level of the toughness can be expressed in thefracture toughness Kc. Note that the fracture toughness Kc can bedetermined by a test method defined in JIS R1607.

According to one embodiment of the present invention, the number ofphotomasks can be reduced as compared to the conventional case withoutusing a complicated technique or a special apparatus. Further, a methodfor manufacturing a liquid crystal display device which is thin,lightweight, and significantly tough can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a top view and a cross-sectional view illustratingone embodiment of the present invention;

FIGS. 2A and 2B are a top view and a cross-sectional view illustratingone embodiment of the present invention;

FIGS. 3A and 3B are circuit diagrams illustrating one embodiment of thepresent invention;

FIGS. 4A-1 and 4B-1 are top views illustrating embodiments of thepresent invention and FIGS. 4A-2 and 4B-2 are cross-sectional viewsillustrating the embodiments of the present invention;

FIGS. 5A to 5C are cross-sectional process views illustrating oneembodiment of the present invention;

FIGS. 6A to 6C are cross-sectional process views illustrating the oneembodiment of the present invention;

FIGS. 7A to 7C are cross-sectional views illustrating one embodiment ofthe present invention;

FIGS. 8A and 8B are a top view and a cross-sectional view illustratingone embodiment of the present invention;

FIGS. 9A to 9C are cross-sectional process views illustrating oneembodiment of the present invention;

FIGS. 10A to 10C are cross-sectional process views illustrating the oneembodiment of the present invention;

FIGS. 11A and 11B are cross-sectional process views illustrating the oneembodiment of the present invention;

FIGS. 12A to 12F are views each illustrating an application example ofan electronic device; and

FIGS. 13A and 13B are views illustrating an application example of anelectronic device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that the present invention is not limited to the followingdescription, and it is easily understood by those skilled in the artthat modes and details of the present invention can be modified in avariety of ways without departing from the spirit and scope of thepresent invention. Therefore, the present invention should not beconstrued as being limited to the description in the followingembodiments. Note that in the structures of the invention describedbelow, the same portions or portions having similar functions aredenoted by the same reference numerals in different drawings, anddescription of such portions is not repeated.

Embodiment 1

In this embodiment, a structural example of a pixel portion included ina liquid crystal display device according to one embodiment of thepresent invention will be described with reference to FIGS. 1A to 6C.

FIG. 3A shows a structure example of a semiconductor device 100 used ina liquid crystal display device. The semiconductor device 100 includes,over a substrate 101, a pixel region 102, a terminal portion 103including m (m is an integer greater than or equal to 1) terminals 105,and a terminal portion 104 including n (n is an integer greater than orequal to 1) terminals 106. The semiconductor device 100 also includes mwirings 212 electrically connected to the terminal portion 103, and nwirings 216 electrically connected to the terminal portion 104. Thepixel region 102 includes a plurality of pixels 110 arranged in matrixof m rows (in the longitudinal direction) x n columns (in the transversedirection). The pixel 110 (i,j) (i is an integer greater than or equalto 1 and less than or equal to m, and j is an integer greater than orequal to 1 and less than or equal to n) in the i-th row and the j-thcolumn is electrically connected to a wiring 212-i and a wiring 216-j.The wiring 212-i is electrically connected to a terminal 105-i, and thewiring 216-j is electrically connected to a terminal 106-j.

The terminal portion 103 and the terminal portion 104 are external inputterminals, and connected to a control circuit provided externally,through a flexible printed circuit (FPC) or the like. Signals suppliedfrom the control circuit provided externally are input to thesemiconductor device 100 through the terminal portions 103 and theterminal portions 104. In FIG. 3A, the terminal portions 103 areprovided on the right external side and the left external side of thepixel region 102, and signals are input from the two portions. Theterminal portions 104 are provided on the upper external side and thelower external side of the pixel region 102, and signals are input fromthe two portions. The signal supply capability is increased by the inputof signals from the two portions; thus, high-speed operation of thesemiconductor device 100 can be easily achieved. Moreover, the influenceof signal delay due to increase in wiring resistance, which is caused byincrease in size or in definition of the semiconductor device 100, canbe reduced. Further, the semiconductor device 100 can have redundancy,so that the reliability of the semiconductor device 100 can beincreased. Note that although FIG. 3A shows the structure in which twoterminal portions 103 and two terminal portions 104 are provided, astructure may be employed in which one terminal portion 103 and oneterminal portion 104 are provided.

FIG. 3B shows a circuit configuration of the pixel 110. The pixel 110includes a transistor 111, a liquid crystal element 112, and a capacitor113. A gate electrode of the transistor 111 is electrically connected tothe wiring 212-i, and one of a source electrode and a drain electrode ofthe transistor 111 is electrically connected to the wiring 216-j. Theother of the source electrode and the drain electrode of the transistor111 is electrically connected to one electrode of the liquid crystalelement 112 and one electrode of the capacitor 113. The other electrodeof the liquid crystal element 112 and the other electrode of thecapacitor 113 are electrically connected to an electrode 114. Thepotential of the electrode 114 may be a fixed potential, e.g., 0V, GND,or a common potential.

The transistor 111 has a function of determining whether an image signalsupplied from the wiring 216-j is input to the liquid crystal element112 or not. When a signal for turning on the transistor 111 is suppliedto the wiring 212-i, an image signal from the wiring 216-j is suppliedto the liquid crystal element 112 through the transistor 111. The lighttransmittance of the liquid crystal element 112 is controlled dependingon the supplied image signal (potential). The capacitor 113 serves as astorage capacitor (also referred to as a Cs capacitor) for keeping thepotential supplied to the liquid crystal element 112. Although thecapacitor 113 is not necessarily provided, when the capacitor 113 isprovided, variation in the potential applied to the liquid crystalelement 112, which is caused by current (off-state current) flowingbetween the source electrode and the drain electrode when the transistor111 is off, can be suppressed.

The capacitance of a storage capacitor provided in a liquid crystaldisplay device is set in consideration of leakage current or the like ofa transistor provided in a pixel portion so that charge can be held fora predetermined period. When the transistor having a highly purifiedoxide semiconductor is used for a semiconductor layer in which a channelregion is formed, it is enough to provide a storage capacitor havingcapacitance that is less than or equal to ⅓, preferably less than orequal to ⅕ of liquid crystal capacitance of each pixel.

A single crystal semiconductor, a polycrystalline semiconductor, amicrocrystalline semiconductor, an amorphous semiconductor, or the likecan be used as a semiconductor for forming a channel of the transistor111. Examples of a semiconductor material include silicon, germanium,silicon germanium, silicon carbide, and gallium arsenide.

Alternatively, an oxide semiconductor may be used as a semiconductor forforming the channel of the transistor 111. The oxide semiconductor maybe either a single crystal oxide semiconductor or a non-single-crystaloxide semiconductor. In the latter case, the non-single-crystal oxidesemiconductor may be amorphous, microcrystalline (nanocrytalline), orpolycrystalline. Further, the oxide semiconductor may have either anamorphous structure including a portion having crystallinity or anon-amorphous structure without a portion having crystallinity. Anamorphous oxide semiconductor can be formed by sputtering with the useof an oxide semiconductor target. A crystalline oxide semiconductor canbe formed while a substrate is heated to a temperature higher than orequal to room temperature in sputtering. As the oxide semiconductor, anoxide semiconductor whose crystallographic axes are aligned can be usedas described in Embodiment 2.

An oxide semiconductor has a wide energy gap of 3.0 eV or more. In atransistor having an oxide semiconductor prepared under appropriateconditions, the off-state current can be lower than or equal to 100 zA(1×10⁻¹⁹ A), further, lower than or equal to 10 zA (1×10⁻²⁰ A),furthermore, lower than or equal to 1 zA (1×10⁻²¹ A) at an operatingtemperature (e.g., at 25° C.).

Thus, the use of an oxide semiconductor for a semiconductor layer of thetransistor 111 in which a channel is formed allows the reduction of thecurrent value in an off state (off-state current value). Accordingly,the retention time of an electric signal such as an image signal can beincreased, and the signal can be held for a long time even if additionalwriting is not carried out. Therefore, the frequency of refreshoperation can be reduced, which contributes to the reduction of powerconsumption. Further, the transistor 111 in which an oxide semiconductoris used for a semiconductor layer can hold a potential supplied to aliquid crystal element, even when a storage capacitor is not provided.

The transistor where an oxide semiconductor is used for a semiconductorlayer in which a channel is formed has a relatively high field-effectmobility, so that high-speed operation is possible. Therefore, by usingsuch a transistor in a pixel portion of a liquid crystal display device,a high-quality image can be provided. In addition, since such atransistor can be provided in each of a driver circuit portion and apixel portion which are provided over one substrate, the number ofcomponents of a liquid crystal display device can be reduced.

Next, a structure example of the pixel 110 illustrated in FIGS. 3A and3B will be described with reference to FIGS. 1A and 1B. FIG. 1A is a topview illustrating a plan structure of the pixel 110, and FIG. 1B is across-sectional view illustrating a layered structure of the pixel 110.Note that dashed-dotted lines A1-A2, B1-B2, and C1-C2 in FIG. 1Acorrespond to cross sections A1-A2, B1-B2, and C1-C2 in FIG. 1B,respectively.

In the transistor 111 described in this embodiment, a drain electrode206 b is partly surrounded by a U-shaped (C-shaped, reversed C-shaped,or horseshoe-shaped) source electrode 206 a. With the source electrodehaving such a shape, a channel width can be sufficiently secured evenwhen the area of a transistor is small; thus, the amount of currentflowing during an on state of the transistor (also referred to as anon-state current) can be increased.

When the parasitic capacitance between a gate electrode 202 and thedrain electrode 206 b electrically connected to a pixel electrode 210 islarge, influence due to feedthrough is readily gained; therefore, thepotential supplied to the liquid crystal element 112 cannot be keptaccurately, which may deteriorate the display quality. As described inthis embodiment, with the structure in which the drain electrode 206 bis partly surrounded by the U-shaped source electrode 206 a, a channelwidth can be sufficiently secured and the parasitic capacitance betweenthe drain electrode 206 b and the gate electrode 202 can be small;whereby the display quality of the liquid crystal display device can beimproved.

In the cross section A1-A2 of FIG. 1B, a layered structure of thetransistor 111 is illustrated. The transistor 111 is a bottom-gatetransistor. In the cross section B1-B2 of FIG. 1B, a layered structureof the capacitor 113 is illustrated. In the cross section C1-C2, alayered structure in a wiring intersection portion of a capacitor wiring203 and the wiring 216 is illustrated.

In the cross section A1-A2, a separation layer 250 is formed over asubstrate 200, a base layer 201 is formed over the separation layer 250,and a gate electrode 202 is formed over the base layer 201. Over thegate electrode 202, a first insulating layer 204 and a semiconductorlayer 205 are formed. Further, the source electrode 206 a and the drainelectrode 206 b are formed over the semiconductor layer 205. Over thesource electrode 206 a and the drain electrode 206 b, a secondinsulating layer 207 is formed in contact with part of the semiconductorlayer 205. The pixel electrode 210 is formed over the second insulatinglayer 207, and electrically connected to the drain electrode 206 bthrough a contact hole 208 formed in the second insulating layer 207.

Part of the first insulating layer 204, part of the semiconductor layer205, and part of the second insulating layer 207 are removed, and thepixel electrode 210 is formed in contact with the first insulating layer204, the semiconductor layer 205, and the second insulating layer 207.In this embodiment, an oxide semiconductor in which the carrierconcentration is significantly reduced (also referred to as an i-type(intrinsic) or substantially i-type oxide semiconductor) is used for thesemiconductor layer 205. The oxide semiconductor can be substantiallyregarded as an insulator in the off state. Therefore, even if the pixelelectrode 210 is in contact with a side surface of an end portion of thesemiconductor layer 205, a problem such as leakage current does notoccur.

Further, since the side surface of the end portion of the semiconductorlayer 205 are covered with the pixel electrode 210, impurities from theoutside such as hydrogen, water, a compound having a hydroxyl group, ahydride, an alkali metal (e.g., sodium, lithium, and potassium), and analkaline earth metal can be prevented from reaching the semiconductorlayer 205 and adversely affecting electric characteristics andreliability of the transistor.

In the cross section B1-B2, the separation layer 250 is formed over thesubstrate 200, the base layer 201 is formed over the separation layer250, and the capacitor wiring 203 is formed over the base layer 201. Thefirst insulating layer 204 and the semiconductor layer 205 are formedover the capacitor wiring 203, and the second insulating layer 207 isformed over the semiconductor layer 205. Further, over the secondinsulating layer 207, the pixel electrode 210 is formed.

A portion where the capacitor wiring 203 and the pixel electrode 210overlap with each other with the first insulating layer 204, thesemiconductor layer 205, and the second insulating layer 207 interposedtherebetween functions as the capacitor 113. The first insulating layer204, the semiconductor layer 205, and the second insulating layer 207function as dielectric layers. Since the dielectric layers formedbetween the capacitor wiring 203 and the pixel electrode 210 has amulti-layer structure, even when a pinhole is formed in one dielectriclayer, the pinhole is covered with another dielectric layer; thus, thecapacitor 113 can function normally. Further, the relative dielectricconstant of an oxide semiconductor is higher than that of silicon oxide,silicon nitride, or the like generally used for an insulating film;thus, the capacitance of the capacitor 113 can be large by using anoxide semiconductor for the semiconductor layer 205.

In the cross section C1-C2, the separation layer 250 is formed over thesubstrate 200, the base layer 201 is formed over the separation layer250, and the capacitor wiring 203 is formed over the base layer 201. Thefirst insulating layer 204 and the semiconductor layer 205 are formedover the capacitor wiring 203. The wiring 216 is formed over thesemiconductor layer 205, and the second insulating layer 207 and thepixel electrode 210 are formed over the wiring 216.

Note that in FIG. 1B, the layers formed over the separation layer 250are hereinafter collectively referred to as an element region 260. Theelement region 260 includes at least the gate electrode 202, the firstinsulating layer 204, the semiconductor layer 205, the source electrode206 a, the drain electrode 206 b, the second insulating layer 207, andthe pixel electrode 210. The element region 260 may further include thebase layer 201 and the wiring 216. Also, the element region 260 mayinclude the wiring 212, an electrode 221, and an electrode 222 which areto be described later, and the like.

Next, a structure of a pixel having a capacitor different from that ofthe pixel 110 in FIGS. 1A and 1B will be described with reference toFIGS. 2A and 2B. FIG. 2A is a top view illustrating a plan structure ofa pixel 120, and FIG. 2B is a cross-sectional view illustrating alayered structure of the pixel 120. Note that dashed-dotted lines A1-A2,B1-B2, and C1-C2 in FIG. 2A correspond to cross sections A1-A2, B1-B2,and C1-C2 in FIG. 2B, respectively.

In the cross section B1-B2, the separation layer 250 is formed over thesubstrate 200, the base layer 201 is formed over the separation layer250, and the capacitor wiring 203 is formed over the base layer 201. Thefirst insulating layer 204 and the semiconductor layer 205 are formedover the capacitor wiring 203, and an electrode 217 is formed over thesemiconductor layer 205. Further, the second insulating layer 207 isformed over the electrode 217, and the pixel electrode 210 is formedover the second insulating layer 207. The pixel electrode 210 iselectrically connected to the electrode 217 through a contact hole 218formed in the second insulating layer 207.

A portion where the capacitor wiring 203 and the electrode 217 overlapwith each other with the first insulating layer 204 and thesemiconductor layer 205 interposed therebetween functions as a capacitor123. The thickness of dielectric layers which are formed between thecapacitor wiring 203 and the electrode 217 in the capacitor 123 can besmaller than that in the capacitor 113 illustrated in FIG. 1B by thethickness of the second insulating layer 207. Thus, the capacitance ofthe capacitor 123 can be larger than that of the capacitor 113.

Note that in FIG. 2B, the layers formed over the separation layer 250are hereinafter collectively referred to as the element region 260. Theelement region 260 includes at least the gate electrode 202, the firstinsulating layer 204, the semiconductor layer 205, the source electrode206 a, the drain electrode 206 b, the second insulating layer 207, andthe pixel electrode 210. The element region 260 may further include thebase layer 201 and the wiring 216. Also, the element region 260 mayinclude the wiring 212, the electrode 221, and the electrode 222 whichare to be described later, and the like.

Next, structure examples of the terminal 105 and the terminal 106 willbe described with reference to FIGS. 4A-1, 4A-2, 4B-1, and 4B-2. FIGS.4A-1 and 4A-2 are a top view and a cross-sectional view illustrating theterminal 105, respectively. A dashed-dotted line D1-D2 in FIG. 4A-1corresponds to a cross section D1-D2 in FIG. 4A-2. FIGS. 4B-1 and 4B-2are a top view and a cross-sectional view illustrating the terminal 106,respectively. A dashed-dotted line E1-E2 in FIG. 4B-1 corresponds to across section E1-E2 in FIG. 4B-2.

In the cross section D1-D2, the separation layer 250 is formed over thesubstrate 200, the base layer 201 is formed over the separation layer250, and the wiring 212 is formed over the base layer 201. The firstinsulating layer 204, the semiconductor layer 205, and the secondinsulating layer 207 are formed over the wiring 212. An electrode 221 isformed over the second insulating layer 207 and electrically connectedto the wiring 212 through a contact hole 219 formed in the firstinsulating layer 204, the semiconductor layer 205, and the secondinsulating layer 207.

In the cross section E1-E2, the separation layer 250 is formed over thesubstrate 200, the base layer 201 is formed over the separation layer250, the first insulating layer 204 is formed over the base layer 201,and the semiconductor layer 205 is formed over the first insulatinglayer 204. The wiring 216 is formed over the semiconductor layer 205,and the second insulating layer 207 is formed over the wiring 216. Anelectrode 222 is formed over the second insulating layer 207, andelectrically connected to the wiring 216 through a contact hole 220formed in the second insulating layer 207.

Next, a manufacturing method of the pixel portion in the liquid crystaldisplay device described using FIGS. 1A and 1C will be described withreference to FIGS. 5A to 5C and FIGS. 6A to 6C. Note that cross sectionsA1-A2, B1-B2, and C1-C2 in FIGS. 5A to 5C and FIGS. 6A to 6C arecross-sectional views taken along the dashed-dotted lines A1-A2, B1-B2,and C1-C2 in FIG. 1A, respectively.

Note that in FIGS. 5A to 5C and FIGS. 6A to 6C, a cross section D1-D2showing a layered structure of the terminal 105 and a cross sectionE1-E2 showing a layered structure of the terminal 106 are additionallyillustrated. In the cross sections D1-D2 and E1-E2, each of D2 and E2corresponds to an edge of the substrate.

First, the separation layer 250 is formed to a thickness of greater thanor equal to 50 nm and less than or equal to 1000 nm, preferably greaterthan or equal to 100 nm and less than or equal to 500 nm, morepreferably greater than or equal to 100 nm and less than or equal to 300nm over the substrate 200.

The substrate 200 may be a glass substrate, a quartz substrate, asapphire substrate, a ceramic substrate, a metal substrate, or the like.Note that such a substrate which is not thin enough to be definitelyflexible enables precise formation of an element such as a transistor.“Not definitely flexible” means that the elastic modulus of thesubstrate is higher than or equivalent to that of a glass substrate usedin generally fabricating a liquid crystal display. In this embodiment,aluminoborosilicate glass is used for the substrate 200.

The separation layer 250 is formed to have a single-layer or layeredstructure using any of elements selected from tungsten (W), molybdenum(Mo), titanium (Ti), tantalum (Ta), niobium (Nb), nickel (Ni), cobalt(Co), zirconium (Zr), zinc (Zn), ruthenium (Ru), rhodium (Rh), palladium(Pd), osmium (Os), iridium (Ir), and silicon (Si), an alloy containingany of the above elements as its main component, and a compoundcontaining any of the above elements as its main component by asputtering method, a plasma CVD method, an application method, aprinting method, or the like.

When the separation layer 250 has a single-layer structure, a layercontaining tungsten, molybdenum, or a mixture of tungsten and molybdenumis preferably formed. Alternatively, a layer containing an oxide or anoxynitride of tungsten, a layer containing an oxide or an oxynitride ofmolybdenum, or a layer containing an oxide or an oxynitride of a mixtureof tungsten and molybdenum is formed. Note that the mixture of tungstenand molybdenum corresponds to an alloy of tungsten and molybdenum, forexample.

In the case where the separation layer 250 has a layered structure, itis preferable that a metal layer and a metal oxide layer be formed as afirst layer and a second layer, respectively. Typically, it ispreferable to form a tungsten layer, a molybdenum layer, or a layercontaining mixture of tungsten and molybdenum as the first layer and toform an oxide, an oxynitride, or a nitride oxide of tungsten,molybdenum, or mixture of tungsten and molybdenum as the second layer.When the metal oxide layer is formed as the second layer, an oxide layer(such as a silicon oxide which can be utilized as an insulating layer)may be formed on the metal layer as the first layer so that an oxide ofthe metal is formed on a surface of the metal layer.

It is also possible to use an amorphous silicon layer containinghydrogen, a layer containing nitrogen, oxygen, hydrogen, or the like(e.g., an amorphous silicon film containing hydrogen, an alloy filmcontaining hydrogen, or an alloy film containing oxygen), or an organicresin, as the separation layer 250.

In this embodiment, a tungsten film with a thickness of 150 nm is usedas the separation layer 250. Note that the tungsten film may be in astate where a surface thereof is oxidized (that is, in a state where atungsten oxide film is formed over the surface of the tungsten film).

The separation layer 250 is a layer mainly for separation of asemiconductor device provided over the base layer 201 from the substrate200 and further has a function of preventing diffusion of an impurityelement from the substrate 200.

Next, over the separation layer 250, an insulating layer serving as thebase layer 201 is formed to a thickness of greater than or equal to 50nm and less than or equal to 300 nm, preferably greater than or equal to100 nm and less than or equal to 200 nm.

The base layer 201 can be formed with a single-layer structure or alayered structure using at least one of the following insulating layers:an aluminum nitride layer, an aluminum oxynitride layer, a siliconnitride layer, a silicon oxide layer, a silicon nitride oxide layer, anda silicon oxynitride layer. The base layer 201 has a function ofpreventing diffusion of an impurity element from the substrate 200 andthe separation layer 250. Note that silicon nitride oxide in thisspecification contains oxygen and nitrogen so that the nitrogen contentis higher than the oxygen content. It is preferred that in the casewhere measurements are performed using Rutherford backscatteringspectrometry (RBS) and hydrogen forward scattering (HFS), siliconnitride oxide be estimated to have a composition of oxygen, nitrogen,silicon, and hydrogen at 5 at. % to 30 at. %, 20 at. % to 55 at. %, 25at. % to 35 at. %, and 10 at. % to 30 at. %, respectively. The baselayer 201 can be formed by a sputtering method, a CVD method, a coatingmethod, a printing method, or the like as appropriate. In the case wherea film containing oxygen such as a silicon nitride oxide film is formedfor the base layer 201, a surface of the separation layer 250 isoxidized so that a metal oxide thin film is formed in the surface of theseparation layer 250 in the film formation. The metal oxide thin film ishandled as the separation layer 250.

In this embodiment, a stack of a silicon nitride layer and a siliconoxide layer is used as the base layer 201. Specifically, a 50-nm-thicksilicon nitride layer is formed over the separation layer 250, and a150-nm-thick silicon oxide layer is formed over the silicon nitridelayer. Note that the base layer 201 may be doped with phosphorus (P) orboron (B).

When a halogen element such as chlorine or fluorine is contained in thebase layer 201, a function of preventing diffusion of an impurityelement from the substrate 200 can be further improved. Theconcentration of a halogen element contained in the base layer 201 ismeasured by secondary ion mass spectrometry (SIMS) and its peak ispreferably greater than or equal to 1×10¹⁵/cm³ and less than or equal to1×10²⁰/cm³.

The base layer 201 may be formed using gallium oxide. Alternatively, thebase layer 201 may have a layered structure of a gallium oxide layer andthe above insulating layer. Gallium oxide is a material which is hardlycharged; therefore, variation in threshold voltage due to charge buildupof the insulating layer can be suppressed.

Next, over the base layer 201, a first conductive layer is formed to athickness of greater than or equal to 100 nm and less than or equal to500 nm, preferably greater than or equal to 200 nm and less than orequal to 300 nm, by a sputtering method, a vacuum evaporation method, ora plating method. Then, a first resist mask is formed over the firstconductive layer and the first conductive layer is partly etched usingthe first resist mask, whereby the gate electrode 202, the capacitorwiring 203, and the wiring 212 are formed.

The first conductive layer for forming the gate electrode 202, thecapacitor wiring 203, and the wiring 212 can be formed to have asingle-layer structure or a layered structure using a metal materialsuch as molybdenum (Mo), titanium (Ti), tungsten (W), tantalum (Ta),aluminum (Al), copper (Cu), chromium (Cr), neodymium (Nd), or scandium(Sc), or an alloy containing any of these elements as its maincomponent. Alternatively, the first conductive layer may be formed usinga conductive metal oxide. As the conductive metal oxide, indium oxide(In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), an indium oxide-tin oxide(In₂O₃—SnO₂, abbreviated to ITO), an indium oxide-zinc oxide(In₂O₃—ZnO), or any of these metal oxides containing silicon oxide canbe used. Still alternatively, a conductive composition containing aconductive macromolecule (also referred to as a conductive polymer) canbe used to form the first conductive layer. As the conductivemacromolecule, a so-called π-electron conjugated conductivemacromolecule can be used. For example, polyaniline or a derivativethereof, polypyrrole or a derivative thereof, polythiophene or aderivative thereof, and a copolymer of two or more of aniline, pyrrole,and thiophene or a derivative thereof can be given.

Since the first conductive layer serves as a wiring, a low resistancematerial such as Al or Cu is preferably used. With use of Al or Cu,signal delay is reduced, so that higher image quality can be expected.Note that Al has low heat resistance, and thus a defect due to hillocks,whiskers, or migration is easily generated. To prevent migration of Al,a layer of a metal having a higher melting point than Al, such as Mo,Ti, or W, is preferably stacked over an Al layer, or an alloy layer ofAl and an element which prevents hillocks, such as Nd, Ti, Si, or Cu, ispreferably used. In the case where a material containing Al is used forthe first conductive layer, the maximum process temperature in a laterstep is preferably set to 380° C. or lower, more preferably 350° C. orlower.

Also in the case where Cu is used for the first conductive layer, inorder to prevent a defect due to migration and diffusion of a Cuelement, a layer of a metal having a higher melting point than Cu, suchas Mo, Ti, or W, is preferably stacked over the first conductive layercontaining Cu. In the case where a material containing Cu is used forthe first conductive layer, the maximum process temperature in a laterstep is preferably set to 450° C. or lower.

In this embodiment, as the first conductive layer, a 5-nm-thick Ti layeris formed over the base layer 201, and a 250-nm-thick Cu layer is formedover the Ti layer. After that, a first resist mask is formed over thefirst conductive layer and the first conductive layer is partly etchedusing the first resist mask, so that the gate electrode 202, thecapacitor wiring 203, and the wiring 212 are formed (see FIG. 5A).

Note that the first resist mask formed over the first conductive layermay be formed by an inkjet method. Formation of the first resist mask byan inkjet method needs no photomask; thus, manufacturing cost can bereduced. The first resist mask is removed after the etching. Descriptionof a process relating to the removal of the first resist mask isomitted.

Note that the etching of the first conductive layer may be either dryetching or wet etching, or both dry etching and wet etching. As anetching gas used for dry etching, a gas containing chlorine (achlorine-based gas such as chlorine (Cl₂), boron trichloride (BCl₃),silicon tetrachloride (SiCl₄), or carbon tetrachloride (CCl₄)) can beused.

For the dry etching, a parallel plate reactive ion etching (RIE) methodor an inductively coupled plasma (ICP) etching method can be used. Theetching conditions are preferably set so that the base layer 201 is notetched as much as possible because the base layer 201 has a function ofpreventing diffusion of an impurity element from the substrate 200.

Then, the first insulating layer 204 serving as a gate insulating layeris formed to a thickness of greater than or equal to 50 nm and less thanor equal to 800 nm, preferably greater than or equal to 100 nm and lessthan or equal to 600 nm over the gate electrode 202, the capacitorwiring 203, and the wiring 212. The first insulating layer 204 can beformed using silicon oxide, silicon nitride, silicon oxynitride, siliconnitride oxide, aluminum oxide, aluminum nitride, aluminum oxynitride,aluminum nitride oxide, tantalum oxide, gallium oxide, yttrium oxide,hafnium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafniumsilicate to which nitrogen is added, hafnium aluminate to which nitrogenis added, or the like. A plasma CVD method, a sputtering method, or thelike can be employed. The first insulating layer 204 is not limited to asingle layer and may be a stack of different layers. For example, thefirst insulating layer 204 may be formed in the following manner: asilicon nitride layer (SiN_(y) (y>0)) is formed by a plasma CVD methodas a gate insulating layer A and a silicon oxide layer (SiO_(x) (x>0))is stacked over the gate insulating layer A as a gate insulating layerB.

In addition to a sputtering method and a plasma CVD method, the firstinsulating layer 204 can be formed by a high-density plasma CVD methodusing microwaves (e.g., a frequency of 2.45 GHz), for example.

In this embodiment, a stack of silicon nitride and silicon oxide is usedas the first insulating layer 204. Specifically, a 50-nm-thick siliconnitride layer is formed over the gate electrode 202, and a 100-nm-thicksilicon oxide layer is formed over the silicon nitride layer.

The first insulating layer 204 also functions as a protective layer.With the structure in which the gate electrode 202 containing Cu iscovered with the insulating layer containing silicon nitride, diffusionof Cu from the gate electrode 202 can be prevented.

In the case where an oxide semiconductor is used for a semiconductorlayer that is formed in a later step, the first insulating layer 204 maybe formed using an insulating material containing a component similar tothat of the oxide semiconductor. In the case where the first insulatinglayer 204 is a stack of different layers, a layer in contact with theoxide semiconductor is formed using an insulating material containing acomponent similar to that of the oxide semiconductor. Such a material iscompatible with the oxide semiconductor, and the use of such a materialfor the first insulating layer 204 allows the interface state betweenthe oxide semiconductor and the first insulating layer 204 to be keptfavorable. Here, “a component similar to that of the oxidesemiconductor” means one or more elements selected from constituentelements of the oxide semiconductor. For example, in the case where theoxide semiconductor is formed using an In—Ga—Zn-based oxidesemiconductor material, gallium oxide is given as an insulating materialcontaining a component similar to that of the oxide semiconductor.

In the case of employing a layered structure, the first insulating layer204 may have a layered structure of a film formed using an insulatingmaterial containing a component similar to that of the oxidesemiconductor and a film containing a material different from thecomponent material of the film.

It is preferable that a high-purity gas in which impurities such ashydrogen, water, a compound having a hydroxyl group, and a hydride areremoved be used as a sputtering gas when the first insulating layer 204is formed. For example, the purity of the high-purity gas in whichimpurities are removed, which is introduced into a sputtering apparatus,is 6N (99.9999%) or higher, preferably 7N (99.99999%) or higher (thatis, the impurity concentration is 1 ppm or lower, preferably 0.1 ppm orlower).

Next, the semiconductor layer 205 is formed over the first insulatinglayer 204.

An oxide semiconductor to be used preferably contains at least indium(In) or zinc (Zn). In particular, In and Zn are preferably contained. Asa stabilizer for reducing change in electric characteristics of atransistor including the oxide semiconductor, gallium (Ga) is preferablyadditionally contained. Tin (Sn) is preferably contained as astabilizer. Hafnium (Hf) is preferably contained as a stabilizer.Aluminum (Al) is preferably contained as a stabilizer.

As another stabilizer, one or plural kinds of a lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium(Lu) may be contained.

As the oxide semiconductor, for example, the following can be used:indium oxide, tin oxide, zinc oxide, a two-component metal oxide such asan In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, aZn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or anIn—Ga-based oxide, a three-component metal oxide such as anIn—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-basedoxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, anAl—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide,an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-basedoxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, anIn—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide,an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-basedoxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or anIn—Lu—Zn-based oxide, or a four-component metal oxide such as anIn—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide.

Note that here, for example, an “In—Ga—Zn-based oxide” means an oxidecontaining In, Ga, and Zn as its main components and there is noparticular limitation on the ratio of In, Ga, and Zn. The In—Ga—Z-basedoxide may contain another metal element in addition to In, Ga, and Zn.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), or anyof oxides whose composition is in the neighborhood of the abovecompositions can be used. Alternatively, an In—Sn—Zn-based oxide with anatomic ratio of In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3(=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or any of oxides whosecomposition is in the neighborhood of the above compositions may beused.

Note that one embodiment of the disclosed invention is not limitedthereto, and a material having an appropriate composition may be useddepending on semiconductor characteristics (mobility, threshold,variation, and the like). Further, it is preferable to appropriately setthe carrier concentration, the impurity concentration, the defectdensity, the atomic ratio of a metal element and oxygen, the interatomicdistance, the density, or the like in order to obtain necessarysemiconductor characteristics.

For example, with an In—Sn—Zn-based oxide, high mobility can be realizedrelatively easily. However, even with an In—Ga—Zn-based oxide, mobilitycan be increased by reducing the defect density in the bulk.

Note that for example, the expression “the composition of an oxide withan atomic ratio of In:Ga:Zn=a:b:c (a+b+c=1) is in the neighborhood ofthe composition of an oxide with an atomic ratio of In:Ga:Zn=A:B:C(A+B+C=1)” means that a, b, and c satisfy the following relation:(a-A)²+(b-B)²+(c-C)²≤r². A variable r may be 0.05, for example. The samecan be applied to other oxides.

The oxide semiconductor may be either a single crystal oxidesemiconductor or a non-single-crystal oxide semiconductor. In the lattercase, the non-single-crystal oxide semiconductor may be either amorphousor polycrystalline. Further, the oxide semiconductor may have either anamorphous structure including a portion having crystallinity or anamorphous structure having no crystalline region.

In an oxide semiconductor in an amorphous state, a flat surface can beobtained with relative ease, so that interface scattering of atransistor can be suppressed, and relatively high mobility can beobtained with relative ease.

In an oxide semiconductor having crystallinity, defects in the bulk canbe further reduced and when surface evenness is improved, mobilityhigher than that of an oxide semiconductor layer in an amorphous statecan be realized. In order to improve the surface evenness, the oxidesemiconductor is preferably formed over a flat surface. Specifically,the oxide semiconductor may be formed over a surface with an averagesurface roughness (Ra) of less than or equal to 1 nm, preferably lessthan or equal to 0.3 nm, more preferably less than or equal to 0.1 nm.

Note that Ra in this specification refers to a centerline averageroughness obtained by three-dimensionally expanding a centerline averageroughness defined by JIS B0601 so as to be applied to a plane to bemeasured. The Ra can be expressed as an “average value of absolutevalues of deviations from a reference plane to a designated plane”, andis defined with the following equation.

$\begin{matrix}{R_{a} = {\frac{1}{S_{0}}{\int_{y\; 1}^{y\; 2}{\int_{x1}^{x2}{{{{f( {x,y} )} - Z_{0}}}{dxdy}}}}}} & \lbrack {{EQUATION}\mspace{14mu} 1} \rbrack\end{matrix}$

Note that in Equation 1, S₀ represents the area of a measurement surface(a rectangular region which is defined by four points represented by thecoordinates (x₁, y₁), (x₁, y₂), (x₂, y₁), and (x₂, y₂)), and Zorepresents average height of a measurement surface. Ra can be measuredusing an atomic force microscope (AFM).

As an example of an oxide semiconductor having crystallinity, there isan oxide including a crystal with c-axis alignment (also referred to asa C-Axis Aligned Crystal (CAAC)), which has a triangular or hexagonalatomic arrangement when seen from the direction of an a-b plane, asurface, or an interface. In the crystal, metal atoms are arranged in alayered manner, or metal atoms and oxygen atoms are arranged in alayered manner along the c-axis, and the direction of the a-axis or theb-axis is varied in the a-b plane (the crystal rotates around thec-axis).

In a broad sense, an oxide including a CAAC means a non-single-crystaloxide including a phase which has a triangular, hexagonal, regulartriangular, or regular hexagonal atomic arrangement when seen from thedirection perpendicular to the a-b plane and in which metal atoms arearranged in a layered manner or metal atoms and oxygen atoms arearranged in a layered manner when seen from the direction perpendicularto the c-axis direction.

The CAAC is not a single crystal, but this does not mean that the CAACis composed of only an amorphous component. Although the CAAC includes acrystallized portion (crystalline portion), a boundary between onecrystalline portion and another crystalline portion is not clear in somecases.

In the case where oxygen is included in the CAAC, part of oxygen may besubstituted with nitrogen. The c-axes of individual crystalline portionsincluded in the CAAC may be aligned in one direction (e.g., a directionperpendicular to a surface of a substrate over which the CAAC is formedor to a surface of the CAAC). Alternatively, the normals of the a-bplanes of the individual crystalline portions included in the CAAC maybe aligned in one direction (e.g., a direction perpendicular to asurface of a substrate over which the CAAC is formed or a surface of theCAAC).

The CAAC becomes a conductor, a semiconductor, or an insulator dependingon its composition or the like. The CAAC transmits or does not transmitvisible light depending on its composition or the like.

As an example of such a CAAC, there is a crystal which is formed into afilm shape and has a triangular or hexagonal atomic arrangement whenobserved from the direction perpendicular to a surface of the film or toa surface of a supporting substrate, and in which metal atoms arearranged in a layered manner or metal atoms and oxygen atoms (ornitrogen atoms) are arranged in a layered manner when a cross section ofthe film is observed.

In this embodiment, as the semiconductor layer 205, an In—Ga—Zn-basedoxide semiconductor is formed to a thickness of 30 nm by a sputteringmethod with the use of an In—Ga—Zn—O-based oxide target (see FIG. 5B).

In order that hydrogen, a hydroxyl group, and moisture may be containedas little as possible in the semiconductor layer 205, it is preferablethat the substrate 200 be preheated in a preheating chamber of asputtering apparatus, for pretreatment before formation of thesemiconductor layer 205, so that impurities such as hydrogen or moistureadsorbed onto the substrate 200 and the first insulating layer 204 areeliminated and removed. Note that this preheating treatment may beomitted. Further, this preheating treatment may be similarly performedon the substrate 200 over which components up to and including the gateelectrode 202, the capacitor wiring 203, and the wiring 212 are formed,before the formation of the first insulating layer 204.

The filling rate of the metal oxide target is higher than or equal to90% and lower than or equal to 100%, preferably higher than or equal to95% and lower than or equal to 99.9%. With the use of the metal oxidetarget with a high filling rate, the formed oxide semiconductor layercan have high density.

It is preferable that a high-purity gas in which impurities such ashydrogen, water, a compound having a hydroxyl group, and a hydride areremoved be used as a sputtering gas when the semiconductor layer 205 isformed. For example, the purity of the high-purity gas in whichimpurities are removed, which is introduced into a sputtering apparatus,is 6N (99.9999%) or higher, preferably 7N (99.99999%) or higher (thatis, the impurity concentration is 1 ppm or lower, preferably 0.1 ppm orlower).

The semiconductor layer 205 is preferably formed by a sputtering methodin which impurities such as hydrogen, water, a hydroxyl group, and ahydride are less likely to enter the semiconductor layer. Deposition isperformed in an oxygen gas atmosphere while the substrate heatingtemperature is higher than or equal to 100° C. and lower than or equalto 600° C., preferably higher than or equal to 150° C. and lower than orequal to 550° C., more preferably higher than or equal to 200° C. andlower than or equal to 500° C. Note that when Al is used for a wiringlayer (e.g., the gate electrode 202) formed by etching the firstconductive layer, the substrate temperature is set to lower than orequal to 380° C., preferably lower than or equal to 350° C. Note thatwhen Cu is used for a wiring layer formed by etching the firstconductive layer, the substrate temperature is set to lower than orequal to 450° C. The thickness of the semiconductor layer 205 is greaterthan or equal to 1 nm and less than or equal to 40 nm, preferablygreater than or equal to 3 nm and less than or equal to 20 nm. As thesubstrate heating temperature in deposition is higher, the impurityconcentration of the obtained semiconductor layer 205 is lower. Further,the atomic arrangement in the semiconductor layer 205 is ordered, thedensity thereof is increased, so that a polycrystal or a CAAC is readilyformed. Furthermore, since an oxygen gas atmosphere is employed for thedeposition, an unnecessary atom is not contained in the semiconductorlayer 205 unlike in the case of employing a rare gas atmosphere or thelike, so that a polycrystal or a CAAC is readily formed. Note that amixed gas atmosphere including an oxygen gas and a rare gas may be used.In that case, the percentage of an oxygen gas is higher than or equal to30 vol. %, preferably higher than or equal to 50 vol. %, more preferablyhigher than or equal to 80 vol. %. Note that as the semiconductor layer205 is thinner, a short-channel effect of a transistor is reduced.However, when the semiconductor layer 205 is too thin, the semiconductorlayer 205 is significantly influenced by interface scattering; thus, thefield effect mobility might be decreased.

By heating the substrate during the deposition, the concentration ofimpurities such as hydrogen, moisture, a hydride, or a hydroxyl group inthe semiconductor layer 205 can be reduced. In addition, damage due tosputtering can be reduced. Then, a high-purity gas in which impuritiessuch as hydrogen, water, a compound having a hydroxyl group, and ahydride are removed is introduced into the deposition chamber from whichmoisture remaining therein is being removed, and the semiconductor layer205 is formed with use of the above target.

In order to remove moisture remaining in the deposition chamber, anentrapment vacuum pump such as a cryopump, an ion pump, or a titaniumsublimation pump is preferably used. As an evacuation unit, a turbomolecular pump to which a cold trap is added may be used. In thedeposition chamber which is evacuated with the cryopump, a hydrogenatom, a compound containing a hydrogen atom such as water (H₂O), (morepreferably, also a compound containing a carbon atom), and the like areevacuated, whereby the impurity concentration in the semiconductor layer205 formed in the deposition chamber can be reduced.

An example of the deposition conditions is as follows: the distancebetween the substrate and the target is 100 mm, the pressure is 0.6 Pa,the DC power is 0.5 kW, and an oxygen atmosphere (the flow rate ofoxygen is 100%) is used. Note that a pulsed DC power source ispreferably used, in which case powder substances (also referred to asparticles or dust) that are generated in deposition can be reduced andthe film thickness can be uniform.

Note that it has been pointed out that an oxide semiconductor isinsensitive to impurities and there is no problem when a considerableamount of metal impurities is contained in an oxide semiconductor film,and therefore, soda-lime glass which contains a large amount of alkalimetal such as sodium and is inexpensive can also be used (Kamiya,Nomura, and Hosono, “Carrier Transport Properties and ElectronicStructures of Amorphous Oxide Semiconductors: The present status”, KOTAIBUTSURI (SOLID STATE PHYSICS), 2009, Vol. 44, pp. 621-633). However,such consideration is not appropriate. When the concentration of alkalimetals in the oxide semiconductor is measured by secondary ion massspectroscopy, it is preferred that the sodium (Na) content is 5×10¹⁶cm⁻³ or less, preferably 1×10¹⁶ cm⁻³ or less, more preferably 1×10¹⁵cm⁻³ or less; the lithium (Li) content is 5×10¹⁵ cm⁻³ or less,preferably 1×10¹⁵ cm⁻³ or less; and the potassium (K) content is 5×10¹⁵cm⁻³ or less, preferably 1×10¹⁵ cm⁻³ or less.

An alkali metal and an alkaline earth metal are adverse impurities forthe oxide semiconductor and are preferably contained as little aspossible. Alkali metal, in particular, a Na ion diffuses from aninsulating film to an oxide when the insulating film in contact with theoxide semiconductor is an oxide. In addition, Na cleaves a bond betweenmetal and oxygen or is inserted between the metal-oxygen bond. As aresult, deterioration of transistor characteristics (e.g., the shift ofa threshold value to the negative side (causing the transistor to benormally on) or a decrease in mobility) is caused. In addition, thisalso causes variation in the characteristics. Such a problem issignificant especially in the case where the hydrogen concentration inthe oxide semiconductor is extremely low. Therefore, the concentrationof an alkali metal is strongly required to be set to the aforementionedvalue in the case where the hydrogen concentration in the oxidesemiconductor is lower than or equal to 5×10¹⁹ cm⁻³, particularly in thecase where it is lower than or equal to 5×10¹⁸ cm⁻³.

Even when the semiconductor layer 205 is formed by the method describedabove, the semiconductor layer 205 contains moisture or hydrogen(including a hydroxyl group) as an impurity in some cases. Moisture orhydrogen easily forms donor levels and thus serves as an impurity in theoxide semiconductor. In order to reduce impurities such as moisture andhydrogen in the semiconductor layer 205 (dehydrate or dehydrogenate thesemiconductor layer 205), the semiconductor layer 205 may be subjectedto heat treatment for dehydration or dehydrogenation (hereinafterabbreviated to first heat treatment) in a reduced-pressure atmosphere,an inert gas atmosphere such as a nitrogen atmosphere or a rare gasatmosphere, an oxygen gas atmosphere, or the like.

By performing the first heat treatment on the semiconductor layer 205,moisture or hydrogen at the surface of the semiconductor layer 205 andin the semiconductor layer 205 can be eliminated. Specifically, heattreatment may be performed at a temperature higher than or equal to 250°C. and lower than or equal to 750° C., preferably higher than or equalto 400° C. and lower than the strain point of a substrate. For example,heat treatment may be performed at 500° C. for approximately threeminutes to six minutes. When an RTA method is used for the heattreatment, dehydration or dehydrogenation can be performed in a shorttime; therefore, treatment can be performed even at a temperature higherthan the strain point of a glass substrate. Note that when Al is usedfor a wiring layer (e.g., the gate electrode 202) formed by etching thefirst conductive layer, the heat treatment temperature is set to lowerthan or equal to 380° C., preferably lower than or equal to 350° C. Notethat when Cu is used for a wiring layer formed by etching the firstconductive layer, the heat treatment temperature is set to lower than orequal to 450° C.

Note that a heat treatment apparatus is not limited to an electricfurnace, and may include a device for heating an object to be processedby heat conduction or heat radiation from a heating element such as aresistance heating element. For example, a rapid thermal anneal (RTA)apparatus such as a gas rapid thermal anneal (GRTA) apparatus or a lamprapid thermal anneal (LRTA) apparatus can be used. An LRTA apparatus isan apparatus for heating an object to be processed by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As the gas, arare gas like argon or an inert gas which does not react with an objectto be processed by heat treatment, such as nitrogen, is used.

The first heat treatment is performed in a reduced-pressure atmosphereor an inert gas atmosphere such as a nitrogen atmosphere, a heliumatmosphere, a neon atmosphere, or an argon atmosphere. Note that it ispreferable that the above atmosphere do not contain moisture, hydrogen,and the like. The purity of nitrogen, or a rare gas such as helium,neon, or argon which is introduced into a heat treatment apparatus isset to 6N (99.9999%) or higher, preferably 7N (99.99999%) or higher(that is, the impurity concentration is 1 ppm or lower, preferably 0.1ppm or lower).

The semiconductor layer 205 which has been subjected to the first heattreatment may be further subjected to second heat treatment. The secondheat treatment is performed in an oxidizing atmosphere in order tosupply oxygen into the semiconductor layer 205, whereby oxygen vacanciesgenerated in the semiconductor layer 205 in the first heat treatment canbe compensated. Thus, the second heat treatment can be referred to astreatment for supplying oxygen. The second heat treatment may beperformed at a temperature of higher than or equal to 200° C. and lowerthan the strain point of the substrate, and is preferably performed at atemperature of higher than or equal to 250° C. and lower than or equalto 450° C. The treatment time is 3 minutes to 24 hours. As the treatmenttime is increased, the proportion of a crystal region with respect tothat of an amorphous region in the semiconductor layer can be increased.Note that heat treatment for longer than 24 hours is not preferablebecause the productivity is reduced.

The oxidizing atmosphere is an atmosphere containing an oxidizing gas.Note that the oxidizing gas is oxygen, ozone, nitrous oxide, or thelike, and it is preferable that the oxidizing gas do not contain water,hydrogen, and the like. For example, the purity of oxygen, ozone, ornitrous oxide introduced into a heat treatment apparatus is set to 6N(99.9999%) or higher, preferably 7N (99.99999%) or higher (i.e., theimpurity concentration is lower than 1 ppm, preferably lower than 0.1ppm). As the oxidizing atmosphere, an oxidizing gas and an inert gas maybe mixed to be used. In that case, the mixture contains an oxidizing gasat a concentration of greater than or equal to 10 ppm. Further, an inertatmosphere refers to an atmosphere containing an inert gas (such asnitrogen or a rare gas (e.g., helium, neon, argon, krypton, or xenon))as the main component. Specifically, the concentration of a reactive gassuch as an oxidizing gas is lower than 10 ppm.

Note that the second heat treatment can be performed using the same heattreatment apparatus and the same gas as those used for the first heattreatment. It is preferable that the first heat treatment fordehydration or dehydrogenation and the second heat treatment for oxygensupply be successively performed. When the first heat treatment and thesecond heat treatment are successively performed, the productivity of asemiconductor device can be increased.

The semiconductor layer 205 purified by a sufficient reduction inhydrogen concentration, in which defect levels in the energy gap due tooxygen deficiency are reduced as a result of sufficient supply ofoxygen, has a carrier concentration of less than 1×10¹²/cm³, less than1×10¹¹/cm³, or less than 1.45×10¹⁰/cm′. Accordingly, the off-statecurrent (per unit channel width (1 μm) here) at room temperature (25°C.) is 100 zA/μm (1 zA (zeptoampere) is 1×10⁻²¹ A) or lower or 10 zA/μmor lower. The off-state current at 85° C. is 100 zA/μm (1×10⁻¹⁹ A/μm) orlower or 10 zA/μm (1×10⁻²⁰ A/μm) or lower. The transistor 111 withextremely excellent off-state current characteristics can be obtainedwith use of such an oxide semiconductor in which the carrierconcentration is significantly reduced (also referred to as an i-type(intrinsic) or substantially i-type oxide semiconductor).

The electric characteristics of the transistor 111, such as thethreshold voltage and the on-state current, have almost no temperaturedependence. Further, the change in transistor characteristics due tolight deterioration hardly occurs.

Thus, variation in electric characteristics of a transistor including ahighly purified oxide semiconductor in which the carrier concentrationis significantly reduced is suppressed and thus, the transistor iselectrically stable. Accordingly, with the use of an oxide semiconductorhaving stable electric characteristics, a highly reliable liquid crystaldisplay device can be provided.

Note that although the case where the first heat treatment and thesecond heat treatment are performed on the semiconductor layer 205immediately after the formation of the semiconductor layer 205 isdescribed above, the heat treatment may be performed at any timing aslong as it is after the formation of the semiconductor layer 205.

Further, after the formation of the semiconductor layer 205, oxygenadding treatment described below may be first performed on thesemiconductor layer 205, and then the first heat treatment may beperformed to eliminate hydrogen, a hydroxyl group, or moisture containedin the oxide semiconductor and simultaneously to allow the oxidesemiconductor to be crystallized. The crystallization may be performedin an additional heat treatment performed later. Through suchcrystallization or recrystallization process, the crystallinity of thesemiconductor layer 205 can be further improved.

Here, the “oxygen adding treatment” means that oxygen (which includes atleast one of an oxygen radical, an oxygen atom, and an oxygen ion) isadded to a bulk of the semiconductor layer 205. Note that the term“bulk” is used in order to clarify that oxygen is added not only to asurface of a thin film but also to the inside of the thin film. Further,“oxygen doping” includes “oxygen plasma doping” in which oxygen which ismade to be plasma is added to a bulk. When oxygen adding treatment isperformed, the amount of oxygen contained in the semiconductor layer 205can be made larger than that in the stoichiometric ratio. Further, afterthe formation of the second insulating layer 207 in a subsequent step,the second insulating layer 207 may be subjected to oxygen addingtreatment, whereby the amount of oxygen in the second insulating layer207 can be made larger than that in the stoichiometric ratio. Byperforming the oxygen adding treatment and then heat treatment on thesecond insulating layer 207, oxygen in the second insulating layer 207can be transported to the semiconductor layer 205 to compensate oxygenvacancies in the semiconductor layer 205 efficiently.

The oxygen adding treatment is preferably performed by an inductivelycoupled plasma (ICP) method, using oxygen plasma excited by microwaves(with a frequency of 2.45 GHz, for example).

Note that the oxygen adding treatment can also be referred to astreatment for supplying oxygen because it is performed so that theamount of oxygen in the semiconductor layer 205, the second insulatinglayer 207, or the like is larger than that in the stoichiometric ratio.The excess oxygen exists mainly between lattices. When the concentrationof oxygen is set to higher than or equal to 1×10¹⁶/cm³ and lower than orequal to 2×10²⁰/cm³, excess oxygen can be contained in the oxidesemiconductor without causing crystal distortion or the like.

Next, a second conductive layer is formed over the semiconductor layer205, a second resist mask is formed over the second conductive layer,and the second conductive layer is partly etched using the second resistmask, so that the source electrode 206 a, the drain electrode 206 b, andthe wiring 216 are formed.

The second conductive layer for forming the source electrode 206 a, thedrain electrode 206 b, and the wiring 216 can be formed to have asingle-layer structure or a layered structure using a metal such asmolybdenum (Mo), titanium (Ti), tungsten (W), tantalum (Ta), aluminum(Al), copper (Cu), chromium (Cr), neodymium (Nd), or scandium (Sc), oran alloy containing any of these as its main component. Alternatively,the second conductive layer may be formed using a conductive metaloxide. As the conductive metal oxide, indium oxide (In₂O₃), tin oxide(SnO₂), zinc oxide (ZnO), an indium oxide-tin oxide (In₂O₃—SnO₂,abbreviated to ITO), an indium oxide-zinc oxide (In₂O₃—ZnO), or any ofthese metal oxide materials containing silicon oxide can be used. Stillalternatively, a conductive composition containing a conductivemacromolecule (also referred to as a conductive polymer) can be used toform the second conductive layer. As the conductive macromolecule, aso-called π-electron conjugated conductive macromolecule can be used.For example, polyaniline or a derivative thereof, polypyrrole or aderivative thereof, polythiophene or a derivative thereof, and acopolymer of two or more of aniline, pyrrole, and thiophene or aderivative thereof can be given.

Since the second conductive layer serves as a wiring, a low resistancematerial such as Al or Cu is preferably used. With use of Al or Cu,signal delay is reduced, so that higher image quality can be expected.Note that Al has low heat resistance, and thus a defect due to hillocks,whiskers, or migration is easily generated. To prevent migration of Al,a layer of a metal material having a higher melting point than Al, suchas Mo, Ti, or W, is preferably stacked over an Al layer, or an alloylayer of Al and an element which prevents hillocks, such as Nd, Ti, Si,or Cu, is preferably used. In the case where a material containing Al isused for the second conductive layer, the maximum process temperature ina later step is preferably set to 380° C. or lower, more preferably 350°C. or lower.

Also in the case where Cu is used for the second conductive layer, inorder to prevent a defect due to migration and diffusion of a Cuelement, a layer of a metal material having a higher melting point thanCu, such as Mo, Ti, or W, is preferably stacked over the secondconductive layer containing Cu. In the case where a material containingCu is used for the second conductive layer, the maximum processtemperature in a later step is preferably set to 450° C. or lower.

In this embodiment, as the second conductive layer, a 5-nm-thick Tilayer is formed over the semiconductor layer 205, and a 250-nm-thick Culayer is formed over the Ti layer. After that, a second resist mask isformed over the second conductive layer and the second conductive layeris partly etched using the second resist mask, so that the sourceelectrode 206 a, the drain electrode 206 b, and the wiring 216 areformed (see FIG. 5C).

Note that the second resist mask formed over the second conductive layermay be formed by an inkjet method. Formation of the second resist maskby an inkjet method needs no photomask; thus, manufacturing cost can bereduced. The second resist mask is removed after the etching.Description thereof is omitted.

Then, over the source electrode 206 a, the drain electrode 206 b, thewiring 216, and the semiconductor layer 205, the second insulating layer207 is formed (see FIG. 6A). The second insulating layer 207 can beformed using a material and a method similar to those of the firstinsulating layer 204 or the base layer 201. A sputtering method ispreferably employed for forming the second insulating layer 207 in termsof low possibility of entry of hydrogen and an impurity congainghydrogen. If hydrogen is contained in the second insulating layer 207,hydrogen might enter the semiconductor layer or extract oxygen in thesemiconductor layer. Therefore, it is important to form the secondinsulating layer 207 by a method in which hydrogen and impuritiescontaining hydrogen are not contained therein.

For the second insulating layer 207, an inorganic insulating materialsuch as silicon oxide, silicon oxynitride, hafnium oxide, aluminumoxide, or gallium oxide can be used. Gallium oxide is a material whichis hardly charged; therefore, variation in threshold voltage due tocharge buildup of the insulating layer can be suppressed. Note that inthe case where an oxide semiconductor is used for the semiconductorlayer 205, a metal oxide layer containing the same kind of component asthe oxide semiconductor may be formed as the second insulating layer 207or stacked over the second insulating layer 207.

In this embodiment, as the second insulating layer 207, a 200-nm-thicksilicon oxide layer is formed by a sputtering method. The substratetemperature in deposition may be higher than or equal to roomtemperature and lower than or equal to 300° C. and in this embodiment,is 100° C. The silicon oxide layer can be formed by a sputtering methodin a rare gas (typically, argon) atmosphere, an oxygen atmosphere, or amixed atmosphere containing a rare gas and oxygen. As a target, asilicon oxide or silicon can be used. For example, with use of siliconfor the target, a silicon oxide layer can be formed by sputtering in anatmosphere containing oxygen.

In order to remove moisture remaining in the deposition chamber at thetime of formation of the second insulating layer 207, an entrapmentvacuum pump such as a cryopump, an ion pump, or a titanium sublimationpump is preferably used. For example, the second insulating layer 207 isformed in the deposition chamber evacuated using a cryopump, whereby theimpurity concentration in the second insulating layer 207 can bereduced. Alternatively, as an evacuation unit for removing moistureremaining in the deposition chamber, a turbo molecular pump providedwith a cold trap may be used.

It is preferable that a high-purity gas in which impurities such ashydrogen, water, a compound containing a hydroxyl group, and a hydrideare removed be used as a sputtering gas when the second insulating layer207 is formed. For example, the purity of the high-purity gas in whichimpurities such as hydrogen, water, a compound containing a hydroxylgroup, and a hydride are removed is 6N (99.9999%) or higher, preferably7N (99.99999%) or higher (that is, the impurity concentration is 1 ppmor lower, preferably 0.1 ppm or lower).

Then, third heat treatment may be performed in a reduced pressureatmosphere, an inert gas atmosphere, an oxygen gas atmosphere, or anultra-dry air atmosphere (preferably at higher than or equal to 200° C.and lower than or equal to 600° C., more preferably higher than or equalto 250° C. and lower than or equal to 550° C.). Note that in the casewhere Al is used for one of or both the wiring layer formed by etchingthe first conductive layer and the wiring layer formed by etching thesecond conductive layer, the heat treatment temperature is set to 380°C. or lower, preferably 350° C. or lower. Alternatively, in the casewhere Cu is used for the wiring layer, the heat treatment temperature isset to 450° C. or lower. For example, the third heat treatment may beperformed in a nitrogen atmosphere at 450° C. for one hour. In the thirdheat treatment, part of the semiconductor layer (channel formationregion) is heated in the state where it is in contact with the secondinsulating layer 207, and thus oxygen can be supplied from the secondinsulating layer 207 containing oxygen to the semiconductor layer 205 sothat oxygen vacancies in the semiconductor layer 205 can be reduced.Note that in the atmosphere at the time of the third heat treatment,impurities such as water and hydrogen are preferably reduced as much aspossible as in the deposition chamber where the second insulating layer207 is formed.

Next, a third resist mask is formed over the second insulating layer207, and the second insulating layer 207, the semiconductor layer 205,and the first insulating layer 204 are partly etched. At that time, inpart of a pixel, where a thin film transistor is not formed, part of thesecond insulating layer 207, part of the semiconductor layer 205, andpart of the first insulating layer 204 are removed, resulting information of a pixel opening 225. Further, over the drain electrode 206b, only the second insulating layer 207 is etched and thus a contacthole 208 is formed. Over the wiring 216 in the cross section E1-E2, onlythe second insulating layer 207 is etched and thus the contact hole 220is formed. Over the wiring 212 in the cross section D1-D2, the secondinsulating layer 207, the semiconductor layer 205, and the firstinsulating layer 204 are etched, and thus a contact hole 219 is formed(see FIG. 6B).

Note that the second insulating layer 207, the semiconductor layer 205,and the first insulating layer 204 in the pixel opening (part of apixel, where a thin film transistor is not formed) are not necessarilyetched. However, in the case where a liquid crystal display device isused as a transmissive liquid crystal display device, etching of thesecond insulating layer 207, the semiconductor layer 205, and the firstinsulating layer 204 in the pixel opening improves the transmittance ofa pixel. Accordingly, the pixel transmits light from a backlightefficiently, and thus reduction in power consumption or improvement indisplay quality due to increase in luminance is possible.

For the etching of the second insulating layer 207, the semiconductorlayer 205, and the first insulating layer 204, either dry etching or wetetching or both may be employed. For example, a gas containing chlorine(a chlorine-based gas such as chlorine (Cl₂), boron trichloride (BCl₃),silicon tetrachloride (SiCl₄), or carbon tetrachloride (CCl₄)) can beemployed as an etching gas used for the dry etching.

For the dry etching, a parallel plate reactive ion etching (RIE) methodor an inductively coupled plasma (ICP) etching method can be used. Theetching conditions are preferably set so that the base layer 201 is notetched as much as possible because the base layer 201 has a function ofpreventing diffusion of an impurity element from the substrate 200.

In general, etching of a semiconductor layer and formation of a contacthole in an insulating layer are performed separately by different resistmask formation steps and different etching steps; however, according tothe manufacturing process of this embodiment, the etching of asemiconductor layer and the formation of a contact hole in an insulatinglayer can be performed by one resist mask formation step and one etchingstep. Therefore, not only reduction in the number of photomasks but alsoreduction in the number of resist mask formation steps and the number ofdifferent etching steps can be achieved. Owing to reduction in thenumber of steps, a liquid crystal display device can be manufactured atlow cost with high productivity.

In addition, according to the manufacturing process of this embodiment,a resist mask is not directly formed on the semiconductor layer 205.Further, since the channel formation region in the semiconductor layer205 is protected by the second insulating layer 207, moisture is notattached to the channel formation region in the semiconductor layer in aseparation step of a photoresist, a cleaning step, and the like; thus,variation in characteristics of the transistor 111 is reduced and thereliability is increased. Particularly in the case where an oxidesemiconductor is used as the semiconductor layer 205, the above effectis more noticeable.

Next, a third conductive layer is formed over the second insulatinglayer 207 by a sputtering method, a vacuum evaporation method, or thelike, a fourth resist mask is formed over the third conductive layer,and the third conductive layer is partly etched, so that the pixelelectrode 210, the electrode 221, and the electrode 222 are formed. Notethat the third conductive layer is preferably formed to a thickness ofgreater than or equal to 30 nm and less than or equal to 200 nm,preferably greater than or equal to 50 nm and less than or equal to 100nm. Through the above steps, the element region 260 is formed over thesubstrate 200 with the separation layer 250 interposed therebetween (seeFIG. 6C).

For the third conductive layer which is used as the pixel electrodes, alight-transmitting conductive material such as indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium tin oxide (hereinafter referred to as ITO), indium zincoxide, or indium tin oxide to which silicon oxide is added is preferablyused. Alternatively, a material formed of 1 to 10 graphene sheets(corresponding to one layer of graphite) may be used. Stillalternatively, a conductive composition containing a conductivemacromolecule (also referred to as a conductive polymer) can be used toform the third conductive layer. As the conductive macromolecule, aso-called π-electron conjugated conductive macromolecule can be used.For example, polyaniline or a derivative thereof, polypyrrole or aderivative thereof, polythiophene or a derivative thereof, and acopolymer of two or more of aniline, pyrrole, and thiophene or aderivative thereof can be given.

In this embodiment, as the third conductive layer, a 80-nm-thick ITOlayer is formed, and a fourth resist mask is formed and the thirdconductive layer is partly etched, whereby the pixel electrode 210, theelectrode 221, and the electrode 222 are formed.

The pixel electrode 210 is electrically connected to the drain electrode206 b through the contact hole 208. The electrode 221 is electricallyconnected to the wiring 212 through the contact hole 219. The electrode222 is electrically connected to the wiring 216 through the contact hole220.

In the contact hole 219 and the contact hole 220, it is important thatthe wiring 212 and the wiring 216 be not kept exposed and be coveredwith an oxide conductive material such as ITO. Since the wiring 212 andthe wiring 216 are metal layers, if the wiring 212 and the wiring 216are kept exposed, the exposed surfaces are oxidized and contactresistance with a FPC or the like is increased, leading to reduction inreliability. The exposed surfaces of the wiring 212 and the wiring 216are covered with an oxide conductive material such as ITO, wherebyincrease in contact resistance can be prevented and thus the reliabilityof the liquid crystal display device can be improved.

Although not illustrated in the drawing, the transistor provided overthe element region 260 is prone to fracture due to static electricity orthe like; thus, a protective circuit is preferably provided. Theprotective circuit is preferably formed using a nonlinear element.

According to this embodiment, a liquid crystal display device can bemanufactured through fewer photolithography processes as compared withthe conventional case. Consequently, a liquid crystal display device canbe manufactured at low cost with high productivity. Further, the elementregion 260 needed for operating the liquid crystal display device isprovided over the substrate 200 with the separation layer 250 interposedtherebetween; thus, the element region 260 can be separated from thesubstrate 200 and transferred to another support.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 2

In this embodiment, a process example partly different from thatdescribed in Embodiment 1 is described with reference to FIGS. 7A to 7C.Note that the same reference numerals are used for the same parts asthose in Embodiment 1, and specific description of the parts with thesame reference numerals is omitted here. Further in this embodiment, theexample of the steps of only a transistor portion will be described withreference to drawings.

First, as in Embodiment 1, the separation layer 250 is formed over thesubstrate 200 having an insulating surface, a first conductive layer isformed over the separation layer 250, a first resist mask is formed overthe first conductive layer, and then, the first conductive layer isselectively etched using the first resist mask to form the gateelectrode 202.

An insulating layer serving as a base layer may be formed between theseparation layer 250 and the gate electrode 202. In this embodiment, thebase layer 201 is formed. The base layer 201 has a function ofpreventing diffusion of an impurity element (e.g., Na) from thesubstrate 200, and can be formed using a film selected from a siliconoxide film, a silicon oxynitride film, a silicon nitride film, a hafniumoxide film, an aluminum oxide film, a gallium oxide film, and a galliumaluminum oxide film. The structure of the base layer 201 is not limitedto a single-layer structure, and may be a layered structure of aplurality of the above films.

In this embodiment, the deposition temperature of a semiconductor layerformed later is higher than or equal to 200° C. and lower than or equalto 450° C., and the temperature of heat treatment performed after theformation of the semiconductor layer is higher than or equal to 200° C.and lower than or equal to 450° C. Therefore, for the gate electrode202, a layered structure in which copper is a lower layer and molybdenumis an upper layer, or a layered structure in which copper is a lowerlayer and tungsten is an upper layer is employed.

Next, as in Embodiment 1, the first insulating layer 204 is formed overthe gate electrode 202 by a CVD method, a sputtering method, or thelike. FIG. 7A is a cross-sectional view illustrating the structureobtained through the steps up to and including this step.

Then, over the first insulating layer 204, a first semiconductor layeris formed to a thickness of greater than or equal to 1 nm and less thanor equal to 10 nm. In this embodiment, the first semiconductor layer isformed to a thickness of 5 nm in an oxygen atmosphere, an argonatmosphere, or a mixed atmosphere of argon and oxygen under thefollowing conditions: a target for an oxide semiconductor (a target foran In—Ga—Zn-based oxide semiconductor (In₂O₃:Ga₂O₃:ZnO=1:1:2 [molarratio]) is used; the distance between the substrate and the target is170 mm; the substrate temperature is 250° C.; the pressure is 0.4 Pa;and the direct current (DC) power is 0.5 kW.

After that, the first heat treatment is performed with the substrateplaced in a nitrogen atmosphere or a dry air atmosphere. The temperatureof the first heat treatment is set to higher than or equal to 200° C.and lower than or equal to 450° C. In the first heat treatment, heatingis performed for greater than or equal to one hour and less than orequal to 24 hours. Through the first heat treatment, a first crystallinesemiconductor layer 748 a is formed (see FIG. 7B).

Next, a second semiconductor layer is formed to a thickness of greaterthan 10 nm over the first crystalline semiconductor layer 748 a. In thisembodiment, the second semiconductor layer is formed to a thickness of25 nm in an oxygen atmosphere, an argon atmosphere, or a mixedatmosphere of argon and oxygen under the following conditions: a targetfor an oxide semiconductor (a target for an In—Ga—Zn-based oxidesemiconductor (In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio]) is used; thedistance between the substrate and the target is 170 mm; the substratetemperature is 400° C.; the pressure is 0.4 Pa; and the direct current(DC) power is 0.5 kW.

Then, the second heat treatment is performed with the substrate placedin a nitrogen atmosphere or a dry air atmosphere. The temperature of thesecond heat treatment is set to higher than or equal to 200° C. andlower than or equal to 450° C. In the second heat treatment, heating isperformed for greater than or equal to one hour and less than or equalto 24 hours. Through the second heat treatment, a second crystallinesemiconductor layer 748 b is formed (see FIG. 7C).

Subsequent steps are performed according to Embodiment 1. After thesource electrode 206 a, the drain electrode 206 b, and the secondinsulating layer 207 are formed, a step of simultaneously forming afirst opening and a second opening is performed using a single resistmask. The first opening is formed by etching part of the secondinsulating layer 207, which overlaps with the drain electrode 206 b. Thesecond opening is formed by etching part of the second insulating layer207, part of the first crystalline semiconductor layer 748 a, part ofthe second crystalline semiconductor layer 748 b, part of the firstinsulating layer 204, and part of the second insulating layer 207, whichdo not overlap with the source electrode 206 a and the drain electrode206 b. Accordingly, the number of steps can be reduced.

After that, the transistor 111 can be obtained by the steps similar tothose in Embodiment 1. Note that in the case where this embodiment isemployed, the semiconductor layer including a channel formation regionof such a transistor has a layered structure of the first crystallinesemiconductor layer 748 a and the second crystalline semiconductor layer748 b. After the formation of the first crystalline semiconductor layer748 a, the first heat treatment is performed so that the firstcrystalline semiconductor layer 748 a includes a CAAC. Then, the secondcrystalline semiconductor layer 748 b is formed and subjected to thesecond heat treatment, whereby with the first crystalline semiconductorlayer 748 a as a seed, a crystal (CAAC) grows in the second crystallinesemiconductor layer 748 b. Thus, an oxide semiconductor including a CAACcan be formed efficiently.

The transistor having a layered structure of the first crystallinesemiconductor layer and the second crystalline semiconductor layer hasstable electric characteristics. When the transistor is irradiated withlight or subjected to a bias-temperature (BT) test, the amount of changeof threshold voltage of the transistor can be reduced.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 3

In this embodiment, an example of a structure of a semiconductor devicewhich is thin, lightweight, and significantly tough and which ismanufactured in such a manner that the element region 260 formed inEmbodiments 1 and 2 is separated from the substrate 200 and is providedover a different support will be described with reference to FIGS. 8Aand 8B. In addition, an example of a method for manufacturing thesemiconductor device will be described with reference to FIGS. 9A to 9C,FIGS. 10A to 10C, and FIGS. 11A and 11B.

To make description on a cross-sectional view of a liquid crystaldisplay device in FIG. 8B, the cross-sectional view of the elementregion 260 described in Embodiment 1 is used for a cross-sectional viewof a pixel portion 850. Note that description will be made assuming thatthe driving method of the liquid crystal display device in thisembodiment is that of a vertical alignment (VA) mode, and the liquidcrystal display device is a reflective monochrome liquid crystal displaydevice including a liquid crystal material exhibiting a blue phase as aliquid crystal layer.

The VA mode is a method of controlling alignment of liquid crystalmolecules of a liquid crystal display panel, in which liquid crystalmolecules are aligned vertically to a panel surface when no voltage isapplied. Some examples are given as the vertical alignment mode. Forexample, a multi-domain vertical alignment (MVA) mode, a patternedvertical alignment (PVA) mode, an advanced super view (ASV) mode, andthe like can be given. Moreover, it is possible to use a method calleddomain multiplication or multi-domain design, in which a pixel isdivided into some regions (subpixels) and molecules are aligned indifferent directions in their respective regions. The VA mode is amethod of controlling alignment of liquid crystal molecules of a liquidcrystal display panel, in which liquid crystal molecules are alignedvertically to a panel surface when no voltage is applied.

Note that for the liquid crystal display device in this embodiment, a VAmode is used; however, the embodiment of the present invention is notlimited thereto. For example, a driving method such as a TN (twistednematic) mode, an IPS (in-plane-switching) mode, an FFS (fringe fieldswitching) mode, an ASM (axially symmetric aligned micro-cell) mode, anOCB (optically compensated birefringence) mode, an FLC (ferroelectricliquid crystal) mode, or an AFLC (antiferroelectric liquid crystal) modemay alternatively be used.

(Example of Structure of Liquid Crystal Display Device)

FIG. 8A is a plan view of a panel in which the element region 260 and aliquid crystal material 840 are sealed between a first support 800 and asecond support 810 with a sealant 820. FIG. 8B is a cross-sectional viewalong dashed-dotted line M-N in FIG. 8A.

The first support 800 has a fracture toughness of the greater than orequal to 1.5 [MPa·m^(1/2)], and is provided with the element region 260with an adhesive 808 for fixation interposed between the first support800 and the element region 260. The use of a material whose fracturetoughness is greater than or equal to 1.5 [MPa·m^(1/2)] for the firstsupport 800 makes it possible to manufacture a liquid crystal displaydevice which is thin, lightweight, and significantly tough.

The fracture toughness of the second support 810 is greater than orequal to 1.5 [MPa·m^(1/2)]. A fourth conductive layer 814 is provided onone surface of the second support 810, and a polarization filter 860 isprovided over the other surface of the second support 810. The use of amaterial whose fracture toughness is greater than or equal to 1.5[MPa·m^(1/2)] for the second support 810 makes it possible tomanufacture a liquid crystal display device which is thin, lightweight,and significantly tough.

The sealant 820, the liquid crystal material 840, and a first conductivematerial 845 are sandwiched between the first support 800 provided withthe element region 260 and the second support 810 provided with thefourth conductive layer 814. Note that the sealant 820 is provided so asto surround the pixel portion 850; thus, the liquid crystal material 840does not leak outside the sealant 820. The first conductive material 845is provided to electrically connect the wiring 212 formed in the elementregion 260 and the fourth conductive layer 814 formed on the secondsupport. Thus, alignment of the liquid crystal material 840 can bechanged by application of voltage between the fourth conductive layer814 and the pixel electrode 210.

An input terminal 880 is provided outside the region surrounded by thesealant 820 provided over the first support 800, and external wirings870 a and 870 b are connected to the wiring 216 in the element region260 through a second conductive material 855. The external wirings 870 aand 870 b each have a function of externally supplying power and signalsnecessary for operating the liquid crystal display device through thesecond conductive material 855.

With the above structure, a liquid crystal display device which is thin,lightweight, and less prone to fracture can be manufactured.

<Method for Manufacturing Liquid Crystal Display Device>

Next, an example of a method for manufacturing the liquid crystaldisplay device described above will be described with reference to FIGS.9A to 9C, FIGS. 10A to 10C, and FIGS. 11A and 11B. Note that amanufacturing process of the liquid crystal display device in thisembodiment will be described by dividing it into a “step of providing anelement region over a first support”, a “step of forming a secondsupport”, and a “step of sealing a liquid crystal layer”.

<Step of Providing Element Region Over First Support>

First, a temporary supporting base 902 is bonded to a surface of theelement region 260 in Embodiment 1, which is formed over the substrate200 with the separation layer 250 interposed therebetween, with the useof an adhesive 900 for separation, and then the element region 260 isseparated from the substrate 200 and transferred to the temporarysupporting base 902 (see FIG. 9A). Note that in this specification, thisstep of separating the element region 260 along the separation layer 250and transferring the element region 260 to the temporary supporting base902 is referred to as a transfer step.

As the adhesive 900 for separation, an adhesive which can be removedfrom the temporary supporting base 902 and the element region 260 asnecessary, such as an adhesive which is soluble to water or an organicsolvent or an adhesive which can be plasticized by ultraviolet lightirradiation. The adhesive 900 for separation is preferably formed to bethin and have a uniform thickness using any of coating machines such asa spin coater, a slit coater, a gravure coater, and a roll coater, orany of printing machines such as a flexible printing machine, an offsetprinting machine, a gravure printing machine, a screen printing machine,and an inkjet machine.

As the temporary supporting base 902, a tape whose adhesion of a surfacecan be arbitrarily decreased, such as a UV separation tape and a thermalseparation tape, can be used. Alternatively, a glass substrate, a quartzsubstrate, a sapphire substrate, a ceramic substrate, a metal substrate,a plastic substrate, or the like may be used. Note that in the casewhere the tape whose adhesion of a surface can be arbitrarily decreasedis used, the adhesive 900 for separation is not necessarily requiredseparately. In the case where a plastic substrate is used as thetemporary supporting base 902, a plastic substrate having heatresistance high enough to withstand the temperature of a processperformed later is preferably used.

Note that there is no particular limitation on the method for bondingthe temporary supporting base 902 to the element region 260. When aflexible material such as the tape is used as the temporary supportingbase 902, a device which can perform bonding using a roller (alsoreferred to as a roll laminator) may be used, for example. Accordingly,the element region 260 and the temporary supporting base 902 can bereliably bonded to each other without air bubbles and the liketherebetween.

In this embodiment, an adhesive which is cured by ultraviolet lightirradiation and is soluble to water (hereinafter referred to as awater-soluble adhesive) is used as the adhesive 900 for separation, andis lightly applied to a surface of the element region 260 with a spincoating apparatus, and curing treatment is performed. After that, a UVseparation tape (a tape whose adhesion can be weakened by UVirradiation) as a temporary supporting base is adhered to the adhesive900 for separation with the use of a roll laminator.

Any of various methods can be used as appropriate to separate theelement region 260 from the substrate 200. For example, in the casewhere the separation layer 250 is formed using an element selected fromtungsten (W), molybdenum (Mo), titanium (Ti), tantalum (Ta), niobium(Nb), nickel (Ni), cobalt (Co), zirconium (Zr), zinc (Zn), ruthenium(Ru), rhodium(Rh), palladium (Pd), osmium (Os), iridium (Ir), andsilicon (Si), or an alloy or compound which contains any of the aboveelements as a main component, and where a metal oxide film is formed inthe surface of the separation layer 250 (for example, in the case wherea film containing oxygen is formed as the base layer 201 after formationof the separation layer 250 as described in Embodiment 1), the metaloxide film is crystallized to be embrittled and force (force to separatethe temporary supporting base 902 from the substrate 200) is applied, sothat the element region 260 can be separated along the separation layer250.

Further, when an amorphous silicon film containing hydrogen is formed asthe separation layer 250, the amorphous silicon film containing hydrogenis removed by laser light irradiation or etching, so that the elementregion 260 can be separated from the substrate 200. When a filmcontaining nitrogen, oxygen, hydrogen, or the like (for example, anamorphous silicon film containing hydrogen, an alloy film containinghydrogen, an alloy film containing oxygen, or the like) is used as theseparation layer 250, the separation layer 250 is irradiated with laserlight to release nitrogen, oxygen, or hydrogen contained in theseparation layer 250 as a gas, so that separation of the element region260 from the substrate 200 can be promoted. Further, a method in whichthe separation layer 250 is removed by etching with the use of a halogenfluoride gas such as NF₃, BrF₃, or ClF₃ may be used.

In the case where an organic resin is used for the separation layer 250,stress inherent in the organic resin may be utilized for separation.

Further, the separation process can be facilitated by using plural kindsof separation methods described above in combination. Specifically, theseparation can be performed with physical force (by a machine or thelike) after performing laser light irradiation on part of the separationlayer, etching on part of the separation layer with a gas, a solution,or the like, or removal of part of the separation layer with mechanicalforce of a sharp knife, a scalpel, or the like, in order that theseparation layer and the element region can be easily separated fromeach other. In the case where the separation layer 250 is formed to havea layered structure of metal and a metal oxide, the layer to beseparated can be physically separated easily from the separation layerby using, for example, a groove formed by laser light irradiation or ascratch made by a sharp knife, a scalpel, or the like as a trigger.

Physically separating the element region 260 enables separation in alarger area in a shorter time as compared to a method in which theelement region 260 is separated by removing the separation layer with asolution, a gas, or the like. In addition, since a solution and a gasare not used, the level of safety is high. Therefore, as a method forseparating the element region 260 from the substrate 200, the method inwhich force is applied for separation is most advantageous in terms ofproductivity and safety.

In the case where separation is performed with a physical means, theseparation may be performed while a liquid such as water is poured.Thus, an adverse effect on the element region 260 due to staticelectricity caused by separation operation (e.g., a phenomenon in whicha semiconductor element is damaged by static electricity) can besuppressed.

Note that in the case where an oxide semiconductor is used for thesemiconductor layer 205, even when static electricity is produced, thesemiconductor layer 205 can be prevented from receiving damage caused bythe static electricity. This is because an oxide semiconductor has ahigher withstand voltage and a lower possibility of dielectric breakdownthan a general semiconductor layer including a silicon material.

In separation of the element region 260 along the separation layer 250,the substrate 200 is fixed so as not to be moved and bent as much aspossible, which enables suppression of force locally applied to theelement region 260. Accordingly, the element region 260 can be separatedwithout any problem (e.g., the element region 260 is not cracked). As amethod for fixing the substrate 200, for example, a method for fixingthe substrate 200 to a stable base using an adhesive material, a methodfor fixing the substrate 200 using a vacuum chuck, or the like isemployed. It is preferable that the substrate 200 be fixed using avacuum chuck in consideration of trouble for separating the substrate200 and reuse of the substrate 200. Specifically, a vacuum chuck, whichhas a porous surface (also referred to as a porous chuck) is preferablyused because the entire surface of the substrate 200 can be fixed withuniform force.

Note that before providing the adhesive 900 for separation over theelement region 260, fluid-jet cleaning, ultrasonic cleaning, plasmacleaning, UV cleaning, ozone cleaning, or the like is preferablyperformed on the element region 260 so that dust and organic componentsattaching to the surface of the element region 260 are removed.

Next, the first support 800 is bonded to the other surface of theelement region 260 with an adhesive 808 for fixation interposedtherebetween (see FIG. 9B).

As a material of the adhesive 808 for fixation, various curableadhesives, e.g., a light curable adhesive such as a UV curable adhesive,a reactive curable adhesive, a thermal curable adhesive, and ananaerobic adhesive can be used.

The adhesive 808 for fixation is preferably formed to be thin and have auniform thickness using any of coating machines such as a spin coater, aslit coater, a gravure coater, and a roll coater, or any of printingmachines such as a flexible printing machine, an offset printingmachine, a gravure printing machine, a screen printing machine, and aninkjet machine.

For the first support 800, any of various materials having hightoughness (specifically, the fracture toughness is greater than or equalto 1.5 [NIPa·m^(1/2)]□ is used. For example, an organic resin substrate,an organic resin thin film, a metal substrate, a metal thin film, or thelike is used. Thus, a liquid crystal display device which is thin,lightweight, and less prone to fracture even in the case where force isapplied externally, for example, an impact is made or bending isperformed, can be manufactured. Note that various materials having hightoughness generally have high flexibility as well as toughness, so thatthe first support 800 having high toughness can be freely bent. Thethickness of the first support 800 may be determined as appropriatedepending on use application of a liquid crystal display device. Forexample, when a liquid crystal display device is provided while beingbent along a shape such as a curved surface, or is rolled up to becarried, the first support 800 may be thin. When a liquid crystaldisplay device is used under the condition that a load is constantlyapplied, the first support 800 may be thick.

As the organic resin substrate and the organic resin thin film, forexample, a substrate and a thin film including as a component at leastone kind of resin selected from the following resins can be used: apoly(ethylene terephthalate) (PET) resin, a poly(ether sulfone) (PES)resin, a poly(ethylene naphthalate) (PEN) resin, a poly(vinyl alcohol)(PVA) resin, a polycarbonate (PC) resin, a nylon resin, an acrylicresin, a polyacrylonitrile resin, a polyetheretherketone (PEEK) resin, apolystyrene (PS) resin, a polysulfone (PSF) resin, a polyetherimide(PEI) resin, a polyarylate (PAR) resin, a poly(butylene terephthalate)(PBT) rein, a polyimide (PI) resin, a polyamide (PA) resin, a poly(amideimide) (PAI) resin, a polyisobutylene (PIB) resin, a chlorinatedpolyether (CP) resin, a melamine (MF) resin, an epoxy (EP) resin, apoly(vinylidene chloride) (PVdC) resin, a polypropylene (PP) resin, apolyacetal (POM) resin, a fluororesin (polytetrafluoroethylene (PTFE)),a phenol (PF) resin, a furan (FF) resin, an unsaturated polyester resin(fiber reinforced plastic (FRP)), a cellulose acetate (CA) resin, a urea(UF) resin, a xylene (XR) resin, a diallyl phthalate (DAP) resin, apoly(vinyl acetate) (PVAc) resin, a polyethylene (PE) resin, and an ABSresin.

As the metal substrate or the metal thin film, for example, aluminum(Al), titanium (Ti), nickel (Ni), chromium (Cr), molybdenum (Mo),tantalum (Ta), beryllium (Be), zirconium (Zr), gold (Au), silver (Ag),copper (Cu), zinc (Zn), iron (Fe), lead (Pb), or tin (Sn), or asubstrate or a thin film including an alloy containing any of theseelements can be used.

In the case of a reflective liquid crystal display device which displaysimages by utilizing reflection of external light as in this embodiment,the above metal substrate or metal thin film having high visible lightreflectance is preferably selected as a material of the first support800 to be used. In particular, the metal substrate or metal thin filmhaving a thermal expansion coefficient of less than or equal to 20 ppm/°C. is preferred. In the case of a transmissive or transflective liquidcrystal display device which displays images with the use of a backlightas a light source, an organic resin substrate or an organic resin thinfilm having high visible light transmittance is preferably used as thefirst support 800, more preferably, an organic resin substrate or anorganic resin thin film having a thermal expansion coefficient of lessthan or equal to 20 ppm/° C. without retardation (birefringence phasedifference) is used.

Note that the first support 800 has a single layer structure in thisembodiment; however, a protective layer may be formed on a top surfaceor a bottom surface of the first support 800. As the protective layer,an inorganic thin film such as a silicon oxide (SiO₂) film, a siliconnitride (SiN) film, a silicon oxynitride (SiON) film, and a siliconnitride oxide (SiNO) film, a metal film such as an aluminum (Al) film ora magnesium (Mg) film, or an oxide film of any of the metals can beused. In particular, a film with low water vapor permeability, low gaspermeability, and low UV transmittance is preferably used. Theprotective layer is preferably formed by a sputtering method or a plasmaCVD method, for example.

Note that in the case of a transmissive or transflective liquid crystaldisplay device, an inorganic film, a metal oxide film, or the likehaving a relatively high visible light transmittance is preferably used.On the other hand, in the case of a reflective liquid crystal displaydevice, a metal film having a high visible light reflectance ispreferably used.

Alternatively, as the protective layer, a resin having resistance to asolvent may be used. The component of the protective layer may beappropriately selected depending on the kind of a chemical solution usedfor substrate cleaning and the kind of a solvent included in analignment film. The component of the protective layer may beappropriately selected from, for example, a poly(vinyl chloride) (PVC)resin, a poly(vinyl alcohol) (PVA) resin, a polyisobutylene (PIB) resin,an acrylic (methacryl) (PMMA) resin, a cellulose acetate (CA) resin, aurea (UF) resin, a xylene (XR) resin, a diallyl phthalate (DAP) resin, apoly(vinyl acetate) resin (PVAc), a polyethylene (PE) resin, a polyamide(PA) (nylon) resin, a polycarbonate (PC) resin, a chlorinated polyether(CP) resin, a melamine (MF) resin, an epoxy (EP) resin, apoly(vinylidene chloride) (PVdC) resin, a polystyrene (PS) resin, apolypropylene (PP) resin, a polyacetal (POM) resin, a fluororesin(polytetrafluoroethylene (PTFE)), a phenol (PF) resin, a furan (FF)resin, an unsaturated polyester resin (fiber reinforced plastic (FRP)),an ABS rein, and the like. With such a resin for the protective layer,it is possible to prevent a change in quality of the first support dueto the chemical solution used for substrate cleaning and the solventincluded in the alignment film.

The protective layer is preferably formed to be thin and have a uniformthickness using any of coating machines such as a spin coater, a slitcoater, a gravure coater, and a roll coater, or any of printing machinessuch as a flexible printing machine, an offset printing machine, agravure printing machine, a screen printing machine, and an inkjetmachine.

Although the first support 800 is bonded to the other surface of theelement region 260 with the adhesive 808 for fixation interposedtherebetween in this embodiment, when a member in which a fibrous bodyis impregnated with an organic resin (a so-called prepreg) is used asthe first support 800, the organic resin with which the fibrous body isimpregnated has a function of the adhesive 808 for fixation; thus, theelement region 260 and the first support 800 can be bonded to each otherwithout the adhesive 808 for fixation. At this time, as the organicresin for the member, a reactive curable resin, a thermosetting resin, aUV curable resin, or the like which is cured by additional treatment ispreferably used.

In this embodiment, a stainless steel film (a so-called SUS film whichis made of a material which contains iron as a base and to whichchromium, nickel, or the like is added) is used as the first support,and a thermosetting adhesive is lightly applied to a surface of thestainless steel film with a screen printing apparatus. The stainlesssteel film to which the thermosetting adhesive is applied is attached tothe other surface of the element region and curing treatment isperformed.

Note that before providing the adhesive 808 for fixation over the firstsupport 800, fluid-jet cleaning, ultrasonic cleaning, plasma cleaning,UV cleaning, ozone cleaning, or the like is preferably performed on thefirst support 800 so that dust and organic components attaching to thesurface of the first support 800 are removed.

Further, heat treatment may be performed on the first support 800. Bythe heat treatment, moisture and impurities attaching to the firstsupport can be removed. Further, by the heat treatment in a reducedpressure state, moisture and impurities can be removed more efficiently.When the heat treatment is performed, a substrate with heat resistancehigh enough to withstand the heat treatment is preferably used as thefirst support 800.

Note that as for the cleaning method and the heat treatment, any one ofthe above cleaning methods and the heat treatment may be selected or twoor more of the cleaning methods and the heat treatment may be performedin combination. For example, after fluid-jet cleaning is performed toremove dust attaching to the first support 800, ozone cleaning isperformed to remove organic components, and then heat treatment isperformed lastly to remove moisture attaching to and absorbed in thefirst support 800 when the fluid-jet cleaning is performed. In such amanner, dust, organic components and moisture on and in the firstsupport 800 can be effectively removed.

Next, the adhesive 900 for separation and the temporary supporting base902 are removed from the element region 260.

In this embodiment, a water-soluble adhesive and a UV separation tapeare used as the adhesive 900 for separation and the temporary supportingbase 902, respectively. Therefore, UV irradiation treatment is performedto remove the temporary supporting base 902 first, and then the adhesive900 for separation is removed by cleaning with water.

Due to the high toughness, the first support 800 has enough flexibilityto be deformed by application of external stress. Therefore, thetemporary supporting base 902 is preferably separated from the elementregion 260 in the state where a substrate with high rigidity is bondedto the first support 800 with an adhesive material provided between thesubstrate with high rigidity and the first support 800 so thatdeformation or fracture is not produced when a load is applied to thefirst support 800 in separation operation and a later step. When thesubstrate with high rigidity is thus bonded, a manufacturing apparatusused for a glass substrate or the like can be used as it is.

Through the above steps, the first support 800 whose surface is providedwith the element region 260 with the adhesive 808 for fixationinterposed therebetween can be manufactured.

Note that an alignment film is not necessarily provided in a liquidcrystal display device according to this embodiment, in which a liquidcrystal material exhibiting a blue phase is used as a liquid crystallayer; thus, an alignment film is not illustrated in FIGS. 8A and 8B,FIGS. 9A to 9C, FIGS. 10A to 10C, and FIGS. 11A and 11B. However, in thecase of a liquid crystal display device in which a liquid crystalmaterial which does not exhibit a blue phase is used as a liquid crystallayer, an alignment film (for example, an insulating organic materialsuch as polyimide (PI), poly(vinyl alcohol) (PVA), or poly(vinylcinnamate) (PVCi) may be used) may be formed over the element region 260(at least over the pixel electrode 210), and rubbing treatment (forexample, the alignment film is rubbed using a roller or the likeprovided with fiber including rayon fiber, cotton fiber, nylon fiber, orthe like as its main material) may be performed on the alignment film sothat the alignment film has an alignment property.

In the case where a liquid crystal material exhibiting a blue phase isused as a liquid crystal layer, an alignment film is not necessarilyprovided, so that rubbing treatment is also unnecessary. Therefore,electrostatic discharge caused by rubbing treatment can be prevented anddefects and damage of the liquid crystal display device in themanufacturing process can be reduced. Accordingly, there is also anadvantage that the productivity of the liquid crystal display device canbe increased.

<Step of Forming Second Support>

Next, the second support 810 is prepared and the fourth conductive layer814 is provided on one surface of the second support 810 (see FIG. 10A).Note that, in this embodiment, a color filter is not provided becausedescription of a monochrome liquid crystal display device is given;however, a color filter may be provided between the second support 810and the fourth conductive layer 814 when a color liquid crystal displaydevice is manufactured.

In the case of providing color filters, in general, color filters ofthree colors of R, G, and B □R, G, and B correspond to red, green, andblue, respectively) are provided in a pixel portion; however, oneembodiment of the present invention is not limited thereto. For example,color filters of R, G, B, and W (W corresponds to white), or colorfilters of R, G, B, and one or more of yellow, cyan, magenta, and thelike may be provided. Further, the sizes of display regions may bedifferent between respective dots of color elements. As a display methodin the pixel portion, a progressive method, an interlace method, or thelike can be employed.

Further, the second support may be provided with a black matrix(light-blocking layer) or an optical member (optical substrate) such asa retardation member or an anti-reflection member as appropriate.

In this embodiment, the case where the driving method of the liquidcrystal display device is a VA mode is described; thus, a structure fora vertical electric field mode in which the fourth conductive layer 814is formed on one surface of the second support 810 and the liquidcrystal material 840 is sandwiched between the pixel electrode 210 andthe fourth conductive layer 814 is employed. However, in the case of thedriving method for a horizontal electric field mode is employed, thefourth conductive layer 814 is not necessarily provided over the secondsupport 810.

For the second support 810, any of various materials having hightoughness (specifically, the fracture toughness is greater than or equalto 1.5 [MPa·m^(1/2)]□ is used. Since the second support needs to have aproperty of not blocking light which travels to the outside, an organicresin substrate or an organic resin thin film is used. Thus, a liquidcrystal display device which is thin, lightweight, and less prone tofracture even in the case where force is applied externally, forexample, an impact is made or bending is performed, can be manufactured.Note that various materials having high toughness generally have highflexibility as well as toughness, so that the second support 810 havinghigh toughness can be freely bent. The thickness of the second support810 may be determined as appropriate depending on use application of aliquid crystal display device. For example, when a liquid crystaldisplay device is provided while being bent along a shape such as acurved surface, or is rolled up to be carried, the second support 810may be thin. Further, when a liquid crystal display device is used underthe condition that a load is constantly applied, the second support 810may be thin.

As the organic resin substrate and the organic resin thin film, amaterial similar to that for the first support 800 may be used.

Formation of a protective layer on the second support 810, cleaning ofthe second support 810, and the like are similar to those of the firstsupport 800; thus, description thereof is omitted here.

Due to the high toughness, the second support 810 has enough flexibilityto be deformed by application of external stress. Therefore, a substratewith high rigidity is preferably bonded to the second support 810 withan adhesive material provided therebetween so that deformation orfracture is not produced when a load is applied to the second support810 in a later step. When the substrate with high rigidity is thusbonded, a manufacturing apparatus used for a glass substrate or the likecan be used as it is. Note that since the substrate with high rigidityneeds to be separated after the second support 810 is bonded to theelement region 260, a low viscosity adhesive (e.g., silicon rubber) or amaterial whose adhesiveness can be weakened by light irradiation or heattreatment (e.g., a UV separation tape or a thermal separation tape) ispreferably used as an adhesive.

The fourth conductive layer 814 can be formed to have a single-layerstructure or a layered structure using a layer including as its maincomponent a light-transmitting conductive material such as indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, indium tin oxide (hereinafter referred to as ITO),indium zinc oxide, or indium tin oxide to which silicon oxide is added,by a sputtering method, a plasma CVD method, a coating method, aprinting method, or the like.

In this embodiment, a polyimide film is used as the second support 810,and an ITO film is formed as the fourth conductive layer 814 to athickness of 200 nm over the polyimide film by a sputtering method.

Although not illustrated in FIG. 10A, a spacer material (a material forkeeping a distance (a so-called gap) for providing the liquid crystalmaterial 840 between the pixel electrode 210 and the fourth conductivelayer 814 when the first support 800 and the second support 810 arebonded to each other) may be provided on the fourth conductive layer 814as necessary after the fourth conductive layer 814 is formed.

Note that an alignment film is not necessarily provided in the liquidcrystal display device according to this embodiment, in which a liquidcrystal material exhibiting a blue phase is used as a liquid crystallayer; thus, an alignment film is not illustrated in FIG. 10A. However,in the case of a liquid crystal display device in which a liquid crystalmaterial which does not exhibit a blue phase as a liquid crystal phaseis used, an alignment film (for example, an insulating organic materialsuch as polyimide (PI), poly(vinyl alcohol) (PVA), or poly(vinylcinnamate) (PVCi) may be used) may be formed over the fourth conductivelayer 814, and rubbing treatment (for example, the alignment film isrubbed using a roller or the like provided with fiber including rayonfiber, cotton fiber, nylon fiber, or the like as its main component) maybe performed on the alignment film so that the alignment film has analignment property.

Note that although the step of forming the fourth conductive layer 814on the second support 810 follows the step of providing the elementregion 260 over the first support 800 with the adhesive 808 for fixationinterposed therebetween in this embodiment, there in no limitation onthe order of these manufacturing steps. The steps are preferablyperformed concurrently to reduce manufacturing time of a liquid crystaldisplay device.

(Step of Sealing Liquid Crystal Layer)

After the first support 800 whose surface is provided with the elementregion 260 with the adhesive 808 for fixation interposed therebetween isprepared, the sealant 820 is provided on the periphery of the pixelportion 850 so as to surround the pixel portion 850, the firstconductive material 845 is provided over the electrode 221, and then theliquid crystal material 840 is provided over the pixel portion 850 (seeFIG. 10B).

In providing the sealant 820, any of the following printing machinessuch as a flexible printing machine, an offset printing machine, agravure printing machine, a screen printing machine, an inkjet machine,and a dispenser may be used. As the sealant 820, any of various curableadhesives, e.g., a photo-curable adhesive such as a UV curable adhesive,a reactive curable adhesive, a thermosetting adhesive, and an anaerobicadhesive may be used. In view of productivity and the influence onvarious materials used for the liquid crystal display device, aphoto-curable adhesive which does not need cure treatment under a hightemperature condition and is cured in a short time is preferably used.Further, the sealant 820 may include a spacer material.

Note that although only one line of the sealant 820 is provided so as tosurround the pixel portion 850 in FIGS. 8A and 8B and FIGS. 10A to 10C,plural lines of the sealants 820 may be provided. By providing plurallines of the sealants 820, the first support 800 and the second support810 can be firmly bonded to each other.

As the first conductive material 845, a material including a conductiveparticle and an organic resin is used. Specifically, a material in whichconductive particles each having a diameter of several nanometers toseveral tens of micrometers are dispersed in an organic resin is used.As the conductive particles, metal particles of one or more of gold(Au), silver (Ag), copper (Cu), nickel (Ni), platinum (Pt), palladium(Pd), tantalum (Ta), molybdenum (Mo), titanium (Ti), aluminum (Al), andcarbon (C), an insulating particle (such as a glass particle or anorganic resin particle) whose surface is provided with a metal filmcontaining one or more of the above metals, microparticles of silverhalide, or the like can be used. As the organic resin contained in thefirst conductive material 845, one or more of the following can be used:an organic resin serving as a binder of the metal particle, an organicresin serving as a solvent of the metal particle, an organic resinserving as a dispersant of the metal particle, or an organic resinserving as a coating member of the metal particle. Organic resins suchas an epoxy resin or a silicone resin can be given as representativeexamples.

In providing the first conductive material 845, any of the followingprinting machines such as a flexible printing machine, an offsetprinting machine, a gravure printing machine, a screen printing machine,an inkjet machine, and a dispenser may be used.

As the liquid crystal material 840, lyotropic liquid crystal,thermotropic liquid crystal, low molecular liquid crystal, highmolecular liquid crystal, discotic liquid crystal, ferroelectric liquidcrystal, anti-ferroelectric liquid crystal, or the like is used. Notethat the above liquid crystal materials exhibit a nematic phase, acholesteric phase, a cholesteric blue phase, a smectic phase, a smecticblue phase, a cubic phase, a smectic D phase, a chiral nematic phase, anisotropic phase, or the like depending on conditions. A cholesteric bluephase and a smectic blue phase are seen in a liquid crystal materialhaving a cholesteric phase or a smectic phase with a relatively shorthelical pitch of less than or equal to 500 nm. The alignment of theliquid crystal material has a double twist structure and the liquidcrystal material has the order with a pitch of less than or equal to anoptical wavelength. A blue phase is one of liquid crystal phases, whichappears just before a cholesteric phase changes into an isotropic phasewhile temperature of cholesteric liquid crystal is increased.

Since the blue phase used in this embodiment appears only in a narrowtemperature range, a liquid crystal composition in which 5 wt % or moreof a chiral material is mixed is used for the liquid crystal material inorder to broaden the temperature range. As for the liquid crystalcomposition which contains a blue-phase liquid crystal and a chiralmaterial, the response speed is as high as 10 μs to 100 μs; an alignmentfilm is not necessary due to optical isotropy; and viewing angledependence is low. Therefore, quality of display images can be improvedand cost reduction can be achieved.

The specific resistivity of the liquid crystal material is greater thanor equal to 1×10⁹ Ω·cm, preferably greater than or equal to 1×10¹¹ Ω·cm,more preferably greater than or equal to 1×10¹² Ω·cm.

Next, the surface of the second support 810, which is provided with thefourth conductive layer 814, is bonded to the surface of the firstsupport 800, which is provided with the liquid crystal material 840, andcuring treatment is performed on the sealant 820 and the firstconductive material 845 (see FIG. 10C).

The first support 800 and the second support 810 are preferably bondedto each other in a treatment chamber kept under reduced pressure in avacuum bonding apparatus or the like. With such a method, bonding can beperformed without inclusion of air bubbles in the sealant 820 or theliquid crystal material 840, and inclusion of an atmospheric componentin the region surrounded by the sealant 820 can be suppressed.

Note that after the bonding is performed, a process for applyingpressure to one side or both sides of the first support 800 and thesecond support 810 is preferably performed. Thus, the liquid crystalmaterial 840 is uniformly formed in the region surrounded by the sealant820.

The curing treatment is performed through one or plural kinds ofprocesses selected from visible light irradiation, UV light irradiation,and heat treatment depending on material components of the sealant 820and the first conductive material 845 such that the cure states of thesealant 820 and the first conductive material 845 are optimized. In thecase where the sealant 820 and the first conductive material 845 arephoto-curable materials for example, the wavelength, the intensity, andthe time of irradiation light are determined as appropriate depending oncure conditions of the materials. Note that when materials for whichcure conditions are the same (e.g., the sealant 820, and the firstconductive material 845 are both photo-curable materials, and thewavelength and the strength of light for curing are almost the same) areused, the number of times of cure treatment can be reduced, which ispreferable. To improve conductivity of the first conductive material 845and to prevent defective conduction, pressure is preferably applied whenthe first conductive material 845 is cured.

In this embodiment, a method (a dropping method) is employed in whichthe first support 800 and the second support 810 are bonded to eachother after dropping the sealant 820 and the liquid crystal material840. Alternatively, a method (an injecting method) may be used in which,after the sealant 820 is dropped, the first support 800 and the secondsupport 810 are bonded to each other, and then, the liquid crystalmaterial 840 is injected into the region surrounded by the sealant 820utilizing capillary action in a space generated between the firstsupport 800 and the second support 810.

Next, the second conductive material 855 is provided over the electrode222 over the first support 800 (see FIG. 11A).

As the second conductive material 855, for example, a material includinga conductive particle and an organic resin is used. Specifically, amaterial in which conductive particles each having a diameter of severalnanometers to several tens of micrometers are dispersed in an organicresin is used. As the conductive particles, metal particles of one ormore of gold (Au), silver (Ag), copper (Cu), nickel (Ni), platinum (Pt),palladium (Pd), tantalum (Ta), molybdenum (Mo), titanium (Ti), aluminum(Al), and carbon (C), an insulating particle whose surface is providedwith a metal film containing one or more of the above metals,microparticles of silver halide, or a solder material can be used. Inaddition, as the organic resin contained in the second conductivematerial 855, one or more of the following can be used: an organic resinserving as a binder of the metal particle, an organic resin serving as asolvent of the metal particle, an organic resin serving as a dispersantof the metal particle, or an organic resin serving as a coating memberof the metal particle. Organic resins such as an epoxy resin or asilicone resin can be given as representative examples.

In providing the second conductive material 855, any of the followingprinting machines such as a flexible printing machine, an offsetprinting machine, a gravure printing machine, a screen printing machine,an inkjet machine, and a dispenser may be used.

In this embodiment, an epoxy resin mixed with flake-like silverparticles each with a size of several nanometers to several tens ofmicrometers is used as the second conductive material 855.

Next, the external wiring 870 a is provided over the second conductivematerial 855 and a connection process is performed to electricallyconnect the external wiring 870 a and the electrode 222, and apolarizing filter 860 is bonded to the second support 810 (see FIG.11B).

As the external wiring 870 a, for example, a printed wiring board or aflexible printed circuit (FPC) may be used. In the liquid crystaldisplay device in this embodiment, both the base substrate and thecounter substrate have high toughness and the liquid crystal displaydevice may have flexibility; thus, it is preferable that the externalwiring 870 a also have flexibility.

For the connection process, the second conductive material 855 may betreated under a condition for curing the second conductive material 855(visible light irradiation, UV light irradiation, or heat treatment). Toimprove conductivity of the second conductive material 855 and toprevent defective conduction between the electrode 222 and the secondconductive material 855, pressure is preferably applied when the secondconductive material 855 is subjected to the connection process. Notethat the connection process is generally performed using athermocompression bonding apparatus in which heat treatment is performedwhile pressure treatment is performed on the second conductive material855 and the external wiring 870 a.

Note that a light source is not provided in this embodiment becausedescription is made on a reflective liquid crystal display devicetherein; however, a backlight, a sidelight, or the like may be providedas a light source on the first support 800 side in the case of atransmissive or transflective liquid crystal display device.

In addition, it is possible to employ a time-division display method(also called a field-sequential driving method) with use of a pluralityof light-emitting diodes (LEDs) as a backlight. By employing afield-sequential driving method, color display can be performed withoutusing a color filter.

Note that the polarizing filter 860 is provided only on the secondsupport 810 side in this embodiment because description is made on areflective liquid crystal display device therein; however, a polarizingfilter may be also provided on the first support 800 side in the case ofa transmissive or transflective liquid crystal display device. Note thatalthough the polarizing filter 860 is bonded after the electrode 222 andthe external wiring 870 a are connected to each other in thisembodiment, these steps may be performed in reverse order.

The element region 260 required for operation of the liquid crystaldisplay device manufactured through the above process is manufacturedwith the use of a smaller number of photomasks than those used in theconventional case. In addition, the element region 260 is formed overthe first support 800 whose fracture toughness is greater than or equalto 1.5 [MPa·m^(1/2)]. Further, the second support 810 with which theliquid crystal material 840 is sandwiched also has a fracture toughnessof greater than or equal to 1.5 [MPa·m^(1/2)].

Thus, the number of photomasks can be reduced without a complicatedtechnique or a special apparatus, and a liquid crystal display devicewhich is thin, lightweight, and significantly tough can be manufactured.

Further, the use of materials with high flexibility for the firstsupport 800 and the second support 810 enables manufacture of a liquidcrystal display device which can be provided while being bent along ashape such as a curved surface or which can be rolled up to be carried.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 4

In this embodiment, an example of an application mode of a semiconductordevice according to one embodiment of the present invention will bedescribed. A semiconductor device according to one embodiment of thepresent invention can be made flexible by being separated from asubstrate over which the semiconductor device has been formed. Specificexamples of electronic devices including semiconductor devices accordingto embodiments of the present invention will be described below withreference to FIGS. 12A to 12F. The electronic devices indicate a liquidcrystal display device, a television device (also referred to as a TVsimply, a TV receiver, or a television receiver), a cellular phone, andthe like.

FIG. 12A illustrates a display 1201 which includes a supporting base1202 and a display portion 1203. The display portion 1203 is formedusing a flexible substrate, which can realize a lightweight and thindisplay. Further, the display portion 1203 can be bent, and can bedetached from the supporting base 1202 and the display can be mountedalong a curved wall. A flexible display, which is one application modeof a semiconductor device according to one embodiment of the presentinvention, can be manufactured with the use of the semiconductor devicedescribed in the above embodiment for the display portion 1203. Thus,the flexible display can be provided on a curved portion as well as aflat surface; therefore, it can be used for various applications.

FIG. 12B illustrates a display 1211 capable of being wound, whichincludes a display portion 1212. A thin and large-area display capableof being wound, which is one application mode of a semiconductor deviceaccording to one embodiment of the present invention, can bemanufactured with the use of the semiconductor device described in theabove embodiment for the display portion 1212. Since the display 1211capable of being wound is formed using a flexible substrate, the display1211 can be carried by being bent or wound along with the displayportion 1212. Therefore, even in the case where the display 1211 capableof being wound is large, the display 1211 can be carried in a bag bybeing bent or wound.

FIG. 12C illustrates a sheet-type computer 1221 which includes a displayportion 1222, a keyboard 1223, a touch pad 1224, an external connectionport 1225, a power plug 1226, and the like. A thin or sheet-typecomputer can be manufactured, which is one application mode of asemiconductor device according to one embodiment of the presentinvention, with the use of the semiconductor device described in theabove embodiment for the display portion 1222. The display portion 1222is formed using a flexible substrate, which can realize a lightweightand thin computer. Further, the display portion 1222 can be wound andstored in a main body when a portion of the main body of the sheet-typecomputer 1221 is provided with a storage space. Furthermore, by alsoforming the keyboard 1223 to be flexible, the keyboard 1223 can be woundand stored in the storage space of the sheet-type computer 1221 in amanner similar to that of the display portion 1222, which is convenientfor carrying around. The computer can be stored without occupying aspace by being bent when it is not used.

FIG. 12D illustrates a display device 1231 having a 20-inch to 80-inchlarge-sized display portion, which includes a keyboard 1233 that is anoperation portion, a display portion 1232, a speaker 1234, and the like.Since the display portion 1232 is formed using a flexible substrate, thedisplay device 1231 can be carried by being bent or wound with thekeyboard 1233 detached. Further, the keyboard 1233 and the displayportion 1232 can be connected without wires. For example, the displaydevice 1231 can be mounted along a curved wall and can be operated withthe keyboard 1233 without wires.

In the example in FIG. 12D, the semiconductor device described in theabove embodiment is used for the display portion 1232. Thus, a thin andlarge-area display device can be manufactured, which is one applicationmode of a semiconductor device according to one embodiment of thepresent invention.

FIG. 12E illustrates an electronic book 1241 which includes a displayportion 1242, an operating key 1243, and the like. In addition, a modemmay be incorporated in the electronic book 1241. The display portion1242 is formed using a flexible substrate and can be bent or wound.Therefore, the electronic book can also be carried without occupying aspace. Further, the display portion 1242 can display a moving image aswell as a still image such as a character.

In the example in FIG. 12E, the semiconductor device described in theabove embodiment is used for the display portion 1242. Thus, a thinelectronic book can be manufactured, which is one application mode of asemiconductor device according to one embodiment of the presentinvention.

FIG. 12F illustrates an IC card 1251 which includes a display portion1252, a connection terminal 1253, and the like. Since the displayportion 1252 is formed using a flexible substrate to have a lightweightand thin sheet-like shape, it can be attached onto a card surface. Whenthe IC card can receive data without contact, information obtained fromoutside can be displayed on the display portion 1252.

In the example in FIG. 12F, the semiconductor device described in theabove embodiment is used for the display portion 1252. Thus, a thin ICcard can be manufactured, which is one application mode of asemiconductor device according to one embodiment of the presentinvention.

When a semiconductor device according to one embodiment of the presentinvention is used for an electronic device, even in the case whereexternal force such as bending is applied to the electronic device sothat stress is caused thereon, damage of an element such as a transistorcan be suppressed; thus, yield and reliability of the semiconductordevice can be increased.

As described above, the application range of the present invention is sowide that the present invention can be applied to electronic devices andinformation displaying means in a wide variety of fields.

Embodiment 5

In this embodiment, with use of the liquid crystal display devicemanufactured according to Embodiment 3 as a display device whichswitches an image for a left eye and an image for a right eye at highspeed, an example in which a 3D image which is a moving image or a stillimage is seen with dedicated glasses with which videos of the displaydevice are synchronized is described with reference to FIGS. 13A and13B.

FIG. 13A illustrates an external view in which a display device 1311 anddedicated glasses 1301 are connected to each other with a cable 1303. Inthe dedicated glasses 1301, shutters provided in a panel 1302 a for aleft eye and a panel 1302 b for a right eye are alternately opened andclosed, whereby a user can see an image of the display device 1311 as a3D image.

In addition, FIG. 13B is a block diagram illustrating a main structureof the display device 1311 and the dedicated glasses 1301.

The display device 1311 illustrated in FIG. 13B includes a displaycontrol circuit 1316, a display portion 1317, a timing generator 1313, asource line driver circuit 1318, an external operation unit 1322, and agate line driver circuit 1319. A semiconductor device according to oneembodiment of the present invention can be used for the display portion1317. Note that an output signal changes in accordance with operationwith the external operation unit 1322 such as a keyboard.

In the timing generator 1313, a start pulse signal and the like areformed, and a signal for synchronizing an image for a left eye and theshutter of the panel 1302 a for a left eye, a signal for synchronizingan image for a right eye and the shutter of the panel 1302 b for a righteye, and the like are formed.

A synchronization signal 1331 a of the image for a left eye is input tothe display control circuit 1316, so that the image for a left eye isdisplayed on the display portion 1317. At the same time, asynchronization signal 1330 a for opening the shutter of the panel 1302a for a left eye is input to the panel 1302 a for a left eye. Inaddition, a synchronization signal 1331 b of the image for a right eyeis input to the display control circuit 1316, so that the image for aright eye is displayed on the display portion 1317. At the same time, asynchronization signal 1330 b for opening the shutter of the panel 1302b for a right eye is input to the panel 1302 b for a right eye.

Since an image for a left eye and an image for a right eye are switchedat high speed, the display device 1311 preferably employs a successivecolor mixing method (a field sequential method) in which color displayis performed by time division with use of light-emitting diodes (LEDs).

Further, since a field sequential method is employed, it is preferablethat the timing generator 1313 input the synchronization signals 1330 aand 1330 b to the backlight portion of the light-emitting diodes. Notethat the backlight portion includes LEDs of colors of R, G, and B.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

This application is based on Japanese Patent Application serial no.2010-204930 filed with the Japan Patent Office on Sep. 13, 2010, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A liquid crystal display device comprising apixel over a flexible substrate, the pixel comprising: a thin filmtransistor comprising: an oxide semiconductor layer; and a sourceelectrode and a drain electrode which are electrically connected to theoxide semiconductor layer; and a liquid crystal element comprising: apixel electrode electrically connected to one of the source electrodeand the drain electrode of the thin film transistor; and a liquidcrystal material over the pixel electrode, wherein the pixel electrodeis in direct contact with a side surface of the oxide semiconductorlayer in the pixel.